mmc: core: fix possible clock gating issue during voltage switch

During voltage sequence (for UHS SD/SDIO cards), host first sends the
voltage switch command (CMD11) to card and then host must stop the clock
at least for 5ms but currently there is a possibility (if clkgate_delay
is 0) that clock may be gated off immediately after the CMD11 response
from card and then get turned on before 5ms itself. This patch ensures
that clock is gated off at least for 5ms after receiving the card
response for voltage switch command.

Change-Id: I131b3d154adab29bef367c8ce31c2f2edd159fd2
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>