mv643xx_eth: fix NETPOLL build

Joseph Fannin <jfannin@gmail.com> and Takashi Iwai <tiwai@suse.de>
noticed that commit 073a345c


("mv643xx_eth: clarify irq masking and unmasking") broke the
mv643xx_eth build when NETPOLL is enabled, due to it not renaming
one instance of INT_CAUSE_EXT in mv643xx_eth_netpoll().  This patch
takes care of that instance as well.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Cc: Dale Farnsworth <dale@farnsworth.org>
Cc: Joseph Fannin <jfannin@gmail.com>
Cc: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>