drivers/net/smc911x.c: Fix lockdep warning on xmit.

dev_kfree_skb should not be called with irqs disabled, use dev_kfree_skb_irq
instead. The warning caused looks like this:

======================================================
[ INFO: hard-safe -> hard-unsafe lock order detected ]
2.6.28-rc1 #273
------------------------------------------------------
swapper/0 [HC0[0]:SC1[2]:HE0:SE0] is trying to acquire:
 (clock-AF_INET){-..+}, at: [<4015c17c>] _sock_def_write_space+0x28/0xd8

and this task is already holding:
 (&lp->lock){++..}, at: [<4013f230>] _smc911x_hard_start_xmit+0x30/0x4b8
which would create a new lock dependency:
 (&lp->lock){++..} -> (clock-AF_INET){-..+}

Signed-off-by: Will Newton <will.newton@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>