net/virtio: fix Rx and Tx handler selection for ARM32

net/virtio: fix Rx and Tx handler selection for ARM32

On arm32, we were always selecting the simple handler, but it is only
available if neon is present.

This is due to a typo in the name of the config option.
CONFIG_RTE_ARCH_ARM is for Makefiles. One should use RTE_ARCH_ARM.

Fixes: 2d7c37194ee4 ("net/virtio: add NEON based Rx handler")
Cc: stable@dpdk.org
Signed-off-by: Samuel Gauthier <samuel.gauthier@6wind.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Acked-by: Yuanhan Liu <yliu@fridaylinux.org>