rt2x00: checkpatch.pl error fixes for rt2500usb.c

rt2500usb.c:42: ERROR: do not initialise statics to 0 or NULL

Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>