spl: mmc: Rename 'mmc' variable to 'mmcp'

The 'p' suffix makes it more obvious that we are dealing with a pointer
to a (pointer) value that will be returned to its caller.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Nikita Kiryanov <nikita@compulab.co.il>
Tested-by: Michal Simek <michal.simek@xilinx.com>