staging: rtl8192u: Replace mdelay with usleep_range in rtl8192_usb_disconnect

rtl8192_usb_disconnect() is never called in atomic context.

rtl8192_usb_disconnect() is only set as ".disconnect" in
struct usb_driver.

Despite never getting called from atomic context,
rtl8192_usb_disconnect() calls mdelay() to busily wait.
This is not necessary and can be replaced with usleep_range() to
avoid busy waiting.

This is found by a static analysis tool named DCNS written by myself.
And I also manually check it.

Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>