mmc: Align MMC_TRACE with tiny printf

The tiny printf implementation only supports %x format specifier,
it does not support %X . Since it makes little difference whether
the debug output prints hex numbers in capitals or not, change it
to %x and make the MMC_TRACE output work with tiny printf too.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Cc: Simon Glass <sjg@chromium.org>