f_phonet: avoid pskb_pull(), fix OOPS with CONFIG_HIGHMEM

This is similar to what we already do in cdc-phonet.c in the same
situation. pskb_pull() refuses to work with HIGHMEM, even if it is
known that the socket buffer is entirely in "low" memory.

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