net: rocker: fix an incorrect array bounds check

While processing transmit(tx) descriptors in 'tx_consume' routine
the switch emulator suffers from an off-by-one error, if a
descriptor was to have more than allowed(ROCKER_TX_FRAGS_MAX=16)
fragments. Fix an incorrect bounds check to avoid it.

Reported-by: Qinghao Tang <luodalongde@gmail.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>
