bnxt_en: add VXLAN dependency

VXLAN may be a loadable module, and this driver cannot be built-in
in that case, or we get a link error:

drivers/built-in.o: In function `__bnxt_open_nic':
drivers/net/ethernet/broadcom/bnxt/bnxt.c:4581: undefined reference to `vxlan_get_rx_port'

This adds a Kconfig dependency that ensures that either VXLAN is
disabled (which the driver handles correctly), or we depend on
VXLAN itself and disallow built-in compilation when VXLAN is
a module.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: c0c050c5 ("bnxt_en: New Broadcom ethernet driver.")
Acked-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>