b43legacy: Do not return TX_BUSY from op_tx

Never return TX_BUSY from op_tx. It doesn't make sense to return
TX_BUSY, if we can not transmit the packet.
Drop the packet and return TX_OK.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>