We must not wake the TX queue without free TX descriptors.
sca_xmit() expects at least one free descriptor and BUGs otherwise.

Problem reported and fix tested by Bernie Innocenti and Ward Vandewege.
Signed-off-by: NKrzysztof Hałasa <khc@pm.waw.pl>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>