staging: rtl8192u: Replace mdelay with msleep in rtl8192_usb_probe

rtl8192_usb_probe is not called in an interrupt handler
nor holding a spinlock.
The function mdelay in it can be replaced with msleep,
to avoid busy wait.

Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>