[PATCH] rt2x00: Clean disabling of rt73usb_get_tsf

By defining rt73usb_get_tsf to NULL we only
have 1 location that needs to be edited
when rt73usb_get_tsf can be enabled again.
This also reduces the number of #ifdefs in
the code which is also a "good thing"

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