b43: Replace mdelay with usleep_range in b43_radio_2057_init_post

b43_radio_2057_init_post is not called in an interrupt handler
nor holding a spinlock.
The function mdelay in it can be replaced with usleep_range,
to reduce busy wait.

Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>