mmc: Change locking for clk scaling from bh to irqsave

Commit 3e825f9bed299
("mmc: clk-scaling: change locking from irq_save to bh") from msm-3.18
changes the spinlock used by clock scaling logic to use the bh flavor
instead of irqsave in order to avoid the unnecessary irq disable.
Unlike the previous kernel versions, in current mmc driver, tasklet is
removed and mmc_clk_scaling_stop_busy can be invoked from IRQ context.
So, change bh back to irqsave to avoid dead lock.

Change-Id: I5c3674273e81bc89dcffd7dd6a6a4170a58bd6bd
Signed-off-by: Jun Li <liju@codeaurora.org>