qlcnic: Fix build failure due to undefined reference to `vxlan_get_rx_port'

Commit 2b3d7b75

("qlcnic: Add VXLAN Rx offload support") uses
vxlan_get_rx_port() which caused build failure when VXLAN=m.

This patch fixes the build failure by adding dependency on VXLAN
in Kconfig of qlcnic module and use vxlan_get_rx_port() and support
code accordingly.

Signed-off-by: Shahed Shaikh <shahed.shaikh@qlogic.com>
Signed-off-by: David S. Miller <davem@davemloft.net>