rtlwifi: Remove extraneous value return

In usb.c, routine _rtl_usb_init_sw() always returns 0, and rtl_usb_probe()
never checks the returned value. Thus we can change _rtl_usb_init_sw() to
a void routine. In addition, remove the underscore at the beginning of the
name of the routine.

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