drivers/dma/mmp_tdma.c: use gen_pool_dma_alloc() to allocate descriptor

Since gen_pool_dma_alloc() is introduced, we implement it to simplify code.

Signed-off-by: Nicolin Chen <b42378@freescale.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Vinod Koul <vinod.koul@intel.com>
Cc: Grant Likely <grant.likely@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>