rt2800: replace msleep() with usleep_range() on channel switch

msleep(1) can sleep much more time then requested 1ms, this is not good
on channel switch, which we want to be performed fast (i.e. to make scan
faster). Replace msleep() with usleep_range(), which has much smaller
maximum sleeping time boundary.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>