via-velocity: fix netif_receive_skb use in irq disabled section.

2fdac010 ("via-velocity.c: update napi
implementation") overlooked an irq disabling spinlock when the Rx part
of the NAPI poll handler was converted from netif_rx to netif_receive_skb.

NAPI Rx processing can be taken out of the locked section with a pair of
napi_{disable / enable} since it only races with the MTU change function.

An heavier rework of the NAPI locking would be able to perform NAPI Tx
before Rx where I simply removed one of velocity_tx_srv calls.

References: https://bugzilla.redhat.com/show_bug.cgi?id=1022733


Fixes: 2fdac010 (via-velocity.c: update napi implementation)
Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Tested-by: Alex A. Schmidt <aaschmidt1@gmail.com>
Cc: Jamie Heilman <jamie@audible.transient.net>
Cc: Michele Baldessari <michele@acksyn.org>
Cc: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>