Staging: et131x: Properly disable FC in txmac.

FC disable is bit 3 of the txmac ctl register, but commit 6720949d


("Staging: et131x: Kil the txmac type") accidentally changed the code to
set bit 2 instead.

Signed-off-by: Nick Bowler <nbowler@draconx.ca>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>