dm: mmc: Update mmc_get_mmc_dev() to use const *

This function does not modify the device to change it to use const *, so
that callers with a const udevice * can call it without a cast.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>