rtl8187: Fix module so that rmmod/insmod does not error

Due to misunderstanding of the returned values allowed for the tx callback
of mac80211, rtl8187 was using skb's that had been freed. This problem was
triggered when the module was sujected to a rmmod/insmod cycle.

After that was fixed, the modules would not work after the rmmod/insmod cycle
until the USB device was reset.

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