drivers/net/cxgb3/xgmac.c: remove dead code

This patch removes dead code ("tx_xcnt" can never be != 0 at this place)
spotted by the Coverity checker.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>