bnx2x: Remove set but not used variable 'mfw_vn'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/ethernet/broadcom/bnx2x/bnx2x_main.c: In function 'bnx2x_get_hwinfo':
drivers/net/ethernet/broadcom/bnx2x/bnx2x_main.c:11940:10: warning:
 variable 'mfw_vn' set but not used [-Wunused-but-set-variable]

It's never used since introduction.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Sudarsana Reddy Kalluru <skalluru@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>