sfc: Ensure down_write(&filter_sem) and up_write() are matched before calling efx_net_open()

This patch avoids the double up_write to filter_sem if
efx_net_open() fails.

Resolves: 2d432f20

Signed-off-by: Shradha Shah <sshah@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>