mmc: msm_sdcc: fix a type mismatch in msmsdccc_config_dma()

msmsdcc_prep_xfer() function returns a signed integer value but
caller function msmsdcc_config_dma() is storing this value in an unsigned
integer variable which is obviously not the correct thing to do.

CRs-Fixed: 476578
Change-Id: I39159e2f4f73c7f54de359cbe4faba5e902915ff
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>