mwifiex: use usleep_range instead of udelay

For the delay of 10 uSec or more usleep_range is prefered.
Unlike udelay, sleep_range avoids large number of undesired
interrupts.

Ref Documentation/timers/timers-howto.txt

Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>