spi/omap: Fix DMA API usage in OMAP MCSPI driver

Running the latest kernel on the 4430SDP board with DMA API debugging
enabled results in this:

WARNING: at lib/dma-debug.c:803 check_unmap+0x19c/0x6f0()
NULL NULL: DMA-API: device driver tries to free DMA memory it has not allocated
[device address=0x000000008129901a] [size=260 bytes]
Modules linked in:
Backtrace:
[<c003cbe0>] (dump_backtrace+0x0/0x10c) from [<c0278da8>] (dump_stack+0x18/0x1c)
 r7:c1839dc0 r6:c0198578 r5:c0304b17 r4:00000323
[<c0278d90>] (dump_stack+0x0/0x1c) from [<c005b158>] (warn_slowpath_common+0x58/0x70)
[<c005b100>] (warn_slowpath_common+0x0/0x70) from [<c005b214>] (warn_slowpath_fmt+0x38/0x40)
 r8:c1839e40 r7:00000000 r6:00000104 r5:00000000 r4:8129901a
[<c005b1dc>] (warn_slowpath_fmt+0x0/0x40) from [<c0198578>] (check_unmap+0x19c/0x6f0)
 r3:c03110de r2:c0304e6b
[<c01983dc>] (check_unmap+0x0/0x6f0) from [<c0198cd8>] (debug_dma_unmap_page+0x74/0x80)
[<c0198c64>] (debug_dma_unmap_page+0x0/0x80) from [<c01d5ad8>] (omap2_mcspi_work+0x514/0xbf0)
[<c01d55c4>] (omap2_mcspi_work+0x0/0xbf0) from [<c006dfb0>] (process_one_work+0x294/0x400)
[<c006dd1c>] (process_one_work+0x0/0x400) from [<c006e50c>] (worker_thread+0x220/0x3f8)
[<c006e2ec>] (worker_thread+0x0/0x3f8) from [<c00738d0>] (kthread+0x88/0x90)
[<c0073848>] (kthread+0x0/0x90) from [<c005e924>] (do_exit+0x0/0x5fc)
 r7:00000013 r6:c005e924 r5:c0073848 r4:c1829ee0
---[ end trace 1b75b31a2719ed20 ]---

I've no idea why this driver uses NULL for dma_unmap_single instead of
the &spi->dev that is laying around just waiting to be used in that
function - but it's an easy fix.

Also replace this comment with a FIXME comment:
                /* Do DMA mapping "early" for better error reporting and
                 * dcache use.  Note that if dma_unmap_single() ever starts
                 * to do real work on ARM, we'd need to clean up mappings
                 * for previous transfers on *ALL* exits of this loop...
                 */
as the comment is not true - we do work in dma_unmap() functions,
particularly on ARMv6 and above.  I've corrected the existing unmap
functions but if any others are required they must be added ASAP.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>