mtd: amd76xrom: fix oops at boot when resources are not available

For some unknown reasons resources needed by amd76xrom driver can be
unavailable. And instead of returning an error, the driver keeps going
and crash the kernel. This patch fixes the problem by making the driver
return -EBUSY if the resources are not available.

Commit messages tweaked by Artem.

Reported-by: Russell Whitaker <russ@ashlandhome.net>
Signed-off-by: Stanislaw Gruszka <stf_xl@wp.pl>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Cc: stable@kernel.org