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>