forcdeth: increase max_interrupt_work

This eliminates the following often-generated warning from my 64 bit
Opteron SMP test stand:

	eth0: too many iterations (6) in nv_nic_irq

According to the web, the problem is that the forcedeth driver has a
too-low value for max_interrupt_work.  Grepping the kernel I see that
forcedeth has the second lowest value of all ethernet drivers (ie, 6).
Most are in the 20-40 range.  So this patch increases this a bit, from 6
to 15 (at 15 forcedeth becomes the driver with third-lowest
max_interrupt_work value).

My test stand, which used to print out the above warnings repetitively
whenever it was under heavy net load, no longer does so.

Signed-off-by: Joe Korty <joe.korty@ccur.com>
Cc: Ayaz Abdulla <aabdulla@nvidia.com>
Cc: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>