p54: tx refused but queue active

In the mainline kernel, p54usb will fail because the TX queue length can
become < 0. This problem has been reported as Bugzilla #13725. The failure
is expressed by the following message in the logs:

WARNING: at net/mac80211/tx.c:1325 ieee80211_tx+0x23c/0x298 [mac80211]()
Hardware name: HP Pavilion dv2700 Notebook PC
tx refused but queue active

This problem has been recently observed in the wireless-testing tree, where
a full solution is being tested. That fix is too invasive for 2.6.31-rcX,
but the simple change supplied here will prevent the failure.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>