mmc: use pr_* log functions

Use pr_* log functions from Linux.  They can be enabled/disabled
via CONFIG_LOGLEVEL.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>