Staging: dt3155v4l: correcting a pointer mismatch bug and cleanups

The third argument of dma_alloc_coherent() is of type dma_addr_t and is defined
as u64 for x86_64, while the member 'device_base' of struct dma_coherent_mem is
defined as u32.

While at it, clean up the code. Now if dev->dma_mem != NULL all the fields/members
are already properly initialized.

Signed-off-by: Marin Mitov <mitov@issp.bas.bg>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>