staging: wilc1000: use usleep_range() in place of udelay()

Changes to avoid the below checkpatch warning:

'usleep_range is preferred over udelay; see
Documentation/timers/timers-howto.txt;'

Signed-off-by: Ajay Singh <ajay.kathat@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>