mmc: atmel: Silence debug output

This driver generates clearly debugging prints when changing clock
speed, so silence those. Furthermore, the driver generates further
prints in case a command fails to complete. The later case woud be
useful, but for eMMC, command 8 can fail and it's not an error but
a part of the specification. Thus, make this debug() as well.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Andreas Bießmann <andreas.devel@googlemail.com>
[fix checkpatch warnings]
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>