8139cp: Don't blindly enable interrupts

Similar to this commit:

http://kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=d15e9c4d9a75702b30e00cdf95c71c88e3f3f51e



It's not safe in cp_start_xmit to blindly call spin_lock_irq and then
spin_unlock_irq, since it may very well be the case that cp_start_xmit
was called with interrupts already disabled (I came across this bug in
the context of netdump in RedHat kernels, but the same issue holds, for
example, in netconsole). Therefore, replace all instances of
spin_lock_irq and spin_unlock_irq with spin_lock_irqsave and
spin_unlock_irqrestore, respectively, in cp_start_xmit(). I tested this
against a fully-virtualized Xen guest using netdump, which happens to
use the 8139cp driver to talk to the emulated hardware. I don't have a
real piece of 8139cp hardware to test on, so someone else will have to
do that.

Signed-off-by: Chris Lalancette <clalance@redhat.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>