mmc: atmel: Properly fix clock configuration

mmc: atmel: Properly fix clock configuration

Timing issue occurs on eMMC not only when modifying the frequency but
also for all the switch command(CMD6). According to the MMC spec waiting
8 clocks after a switch command would be the thing to do.

This patch allows fixing CPU hang observed when trying to changing the
bus width on a eMMC on SAMA5D4.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Tested-by: Marek Vasut <marex@denx.de> # on DENX MA5D4EV
Acked-by: Marek Vasut <marex@denx.de>
Tested-by: Andreas Bießmann <andreas.devel@googlemail.com> # on atngw100
Acked-by: Andreas Bießmann <andreas.devel@googlemail.com>
[fixed minor checkpatch warning]
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>