mhi: core: Using usleep_range instead of msleep for sleep

msleep is not accurate for small delays.  Use usleep_range
for accuracy.

CRs-Fixed: 1117121
Change-Id: I9a076f843875bd83a21c96d4e4ea0510e82ce352
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>