USB: 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.

Signed-off-by: Rémi Denis-Courmont <remi.denis-courmont@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>