net: Fix misplaced parenthesis in virtio_net.c

Commit 2e57b79c misplaced its
parenthesis and now tx_fifo_errors will only be incremented if an
ENOMEM error is not written to the syslog.

Correct the parenthesis and indentation to the original goal of
counting all non ENOMEM errors and ratelimiting only the messages.

Signed-of-by: Torsten Kaiser <just.for.lkml@googlemail.com>

Signed-off-by: David S. Miller <davem@davemloft.net>