wl1251: fix txop unit

mac80211 uses unit units of 32 usec with txop but wl1251_acx_ac_cfg()
expects it to be usecs. This fortunately didn't cause any severe problems,
only that firmware was using incorrect WMM settings.

Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>