s3cmci: fix direct write to interrupt mask

The clear_imask() call should be used to clear the interrupt mask
register, as it may end up clearing the SDIO interrupt bit if this is
enabled.

Change all writes of zero to SDIIMSK register to use clear_imask() ready
for the SDIO updates.

Signed-off-by: Ben Dooks <ben@simtec.co.uk>
Cc: <linux-mmc@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>