mmc: core: Use usleep_range for delays less than 20ms

msleep of any delay less than 20ms (or 2 jiffies) would
give an unpredictable delay (most of the times as ~20ms).
Use usleep_range in mmc_delay() for delays less than two jiffies.

Change-Id: I7cf22532af2b4048afccf07a5aeba89d808799f0
Signed-off-by: Sujit Reddy Thumma <sthumma@codeaurora.org>