f_phonet: dev_kfree_skb instead of dev_kfree_skb_any in TX callback

Network device TX is never run in IRQ context, and skb is freed outside
of the IRQ-disabling spin lock. So checking for IRQ was a waste of time
here.

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