mmc: Fix static checker warnings

Correct pointer dereferencing check to be more consistent.

Eliminate the below smatch warning:
drivers/mmc/mmc.c:3118 mmc_init_device()
warn: variable dereferenced before check 'm' (see line 3116)

Signed-off-by: Venkatesh Yadav Abbarapu <venkatesh.abbarapu@amd.com>
Reviewed-by: Michal Simek <michal.simek@amd.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>