mtd/omap2: fix dmaengine_slave_config error handling

The newly added dmaengine support in the omap2 nand driver
potentially causes an undefined return value from the
omap_nand_probe function when dmaengine_slave_config
reports an error. Let's handle this by returning the
same error back to the caller.

Without this patch, building omap2plus_defconfig results in:

drivers/mtd/nand/omap2.c: In function 'omap_nand_probe':
drivers/mtd/nand/omap2.c:1154:6: warning: 'err' may be used uninitialized in this function [-Wuninitialized]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Russell King <rmk+kernel@arm.linux.org.uk>
Cc: Grazvydas Ignotas <notasas@gmail.com>