rt2x00: Optimize rt2x00debug_dump_frame when frame dumping is not active

When rt2x00 is compiled with debugging but frame dumping is currently
not active we can avoid the call to do_gettimeofday. Furthermore,
frame dumping is not the default case, mark it as unlikely.

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