mmc: block: fix format string warning

Fixed the following warning (reported by cppcheck):
[drivers/mmc/card/block.c:2149]: (warning) %d in format string (no. 1)
requires 'int' but the argument type is 'unsigned int'.

Signed-off-by: Asaf Vertz <asaf.vertz@tandemg.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>