e1000e: fix e1000_netpoll(), remove extraneous e1000_clean_tx_irq() call

Evgeniy Polyakov noticed that drivers/net/e1000e/netdev.c:e1000_netpoll()
was calling e1000_clean_tx_irq() without taking the TX lock.

David Miller suggested to remove the call altogether: since in this
callpah there's periodic calls to ->poll() anyway which will do
e1000_clean_tx_irq() and will garbage-collect any finished TX ring
descriptors.

This fix solved the e1000e+netconsole crashes i've been seeing:

=============================================================================
BUG skbuff_head_cache: Poison overwritten
-----------------------------------------------------------------------------

INFO: 0xf658ae9c-0xf658ae9c. First byte 0x6a instead of 0x6b
INFO: Allocated in __alloc_skb+0x2c/0x110 age=0 cpu=0 pid=5098
INFO: Freed in __kfree_skb+0x31/0x80 age=0 cpu=1 pid=4440
INFO: Slab 0xc16cc140 objects=16 used=1 fp=0xf658ae00 flags=0x400000c3
INFO: Object 0xf658ae00 @offset=3584 fp=0xf658af00

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: David S. Miller <davem@davemloft.net>