mmc: atmel: Fix clock configuration

After silencing the prints which were generated when reconfiguring the
clock of the SD/MMC bus, surprisingly, the driver stopped working such
that every attempt to use the SD/MMC bus caused the CPU to get totally
stuck hard. It turns out that the prints generated a short delay, which
was necessary for the CPU to reconfigure the clock without getting stuck.
Thus, this patch adds a short delay after the clock configuration instead.

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