natsemi: Consistently use interrupt enable/disable functions

The natsemi drivers include functions for enabling and disabling
interrupts from the chip but these are not used in all code paths.  This
patch changes the code paths that touch the interrupt enable register to
use the functions.  In all cases this adds an extra PCI read to post the
operation but since none of these are in fast paths this shouldn't be
too much of a problem.

Signed-Off-By: Mark Brown <broonie@sirena.org.uk>

Signed-off-by: Jeff Garzik <jeff@garzik.org>