x86, oprom: ensure DF is always cleared

x86, oprom: ensure DF is always cleared

The Option ROM might mess with the EFLAGS register and break assumptions
the C part of coreboot implicitly has, e.g. the state of the direction
flag.

Prevent Option ROMs from confusing coreboot by restoring the old EFLAGS
value after the Option ROMs has finished and always clear the direction
flag before calling the C part of the interrupt handler.

Change-Id: I84663be6681b17f95f48d93f0b730e443336b4a8
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Reviewed-on: http://review.coreboot.org/837
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>