mmc: host: Set max frequency when disabling clock scaling

The mmc host needs to perform at its peak when clock scaling
is disabled, hence switch the frequency to max.

CRs-fixed: 981387
Change-Id: Ie959b8e565ee2dad53cdd9d913bcb8696519d7ca
Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>