f_phonet: fix page offset of first received fragment

We pull one byte (the MAC header) from the first fragment before the
fragment is actually appended. So the socket buffer length is 1, not 0.

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