Phonet: fix accounting race between gprs_writeable() and gprs_xmit()

In the unlikely event that gprs_writeable() and gprs_xmit() check for
writeability at the same, we could stop the device queue forever.

Signed-off-by: Rémi Denis-Courmont <remi.denis-courmont@nokia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>