rt2x00: Optimize get_duration / get_duration_res

We can optimize get_duration and get_duration_res
by making them a macro. They are really simple
calculation handlers so this doesn't matter much.

Often (especially in rt2400pci and rt2500pci,
the arguments are hardcoded, and the result value
is passed into other hardcoded values.

By making the functions a macro GCC can
optimize the entire thing much better.

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