qlcnic: Fix NULL pointer dereference in VF probe path.

o Check for non-NULL set_mac_filter_count function pointer
  before calling it fixes the panic.

This patch fixes regression introduced by patch
"qlcnic: Secondary unicast MAC address support." with
commit id 168e4fb54c11865668ad50eff81b5f2729e0e0f4.

Signed-off-by: Sucheta Chakraborty <sucheta.chakraborty@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>