[e1000] Remove unused variable when ICR register is simply cleared

[e1000] Remove unused variable when ICR register is simply cleared

On reset and close, the ICR register is read to clear any pending
interrupts, but the value is simply ignored.  Avoid assigning the
value to a variable, to inhibit a warning from gcc 4.6.

Also fix a potential race condition in reset routines which clear
interrupts before disabling them.

Reported-by: Ralph Giles <giles@thaumas.net>
Tested-by: Ralph Giles <giles@thaumas.net>
Signed-off-by: Michael Brown <mcb30@ipxe.org>