vmxnet3: fix building without CONFIG_PCI_MSI

Since commit d25f06ea

 "vmxnet3: fix netpoll race condition",
the vmxnet3 driver fails to build when CONFIG_PCI_MSI is disabled,
because it unconditionally references the vmxnet3_msix_rx()
function.

To fix this, use the same #ifdef in the caller that exists around
the function definition.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Neil Horman <nhorman@tuxdriver.com>
Cc: Shreyas Bhatewara <sbhatewara@vmware.com>
Cc: "VMware, Inc." <pv-drivers@vmware.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: stable@vger.kernel.org
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>