rtlwifi: Detect misread of end-point count

In the unlikely event of a misread of the USB end point count, the driver
generates a divide fault. To prevent this, add a check of the value
returned by _rtl_usb_init(). In addition, add some logging to indicate
why the condition occurred.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>