mmc: clk-scaling: change locking from irq_save to bh

This commit changes the spinlock used by clock scaling
logic to use the bh flavor instead of irqsave.
This is done in order to avoid the unnecessary irq
disable while going into clock scaling critical sections.

Change-Id: Id660db89dc531336621f40908fd3a4ad4777a12d
Signed-off-by: Talel Shenhar <tatias@codeaurora.org>