ath5k: correct endianness of frame duration

The ath5k version of ieee80211_generic_frame_duration() returns
an __le16 for standard modes but a cpu-endian int for turbo/half/
quarter rates.  Make it always return cpu-endian values.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Acked-by: Bruno Randolf <br1@einfach.org>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>