mmc: sdhi, mmcif: zboot: Correct clock disable logic

This corrects a logic-error that I made in the original implementation.

An alternate patch would be to just remove these lines and
leave the clock running as it is reconfigured later on during
boot anyway.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>