fs_enet: silence a build warning (unused variable)

Since commit 720a43ef
(drivers:net: Remove unnecessary OOM messages after netdev_alloc_skb)
there is a build warning:

drivers/net/ethernet/freescale/fs_enet/fs_enet-main.c: In function 'tx_skb_align_workaround':
drivers/net/ethernet/freescale/fs_enet/fs_enet-main.c:586:26: warning: unused variable 'fep'

Fix it.

Signed-off-by: Gerhard Sittig <gsi@denx.de>
Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>