nvbios_extend() returns 1 to indicate "extended the array" and 0 to
indicate the array is already big enough.  This is used by the core
shadowing code to prevent re-fetching chunks of the image that have
already been shadowed.

The ACPI fetching code may possibly need to extend this further due
to requiring fetches to happen in 4KiB chunks.

Under certain circumstances (that happen if the total image size is
a multiple of 4KiB), the memory allocated to store the shadow will
already be big enough, causing the ACPI code's nvbios_extend() call
to return 0, which is misinterpreted as a failure.

The fix is simple, accept >= 0 as a successful condition here.  The
core will have already made sure that we're not re-fetching data we
already have.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=89047

v2 (Ben Skeggs):
- dropped hunk which would cause unnecessary re-fetching
- more descriptive explanation
Signed-off-by: Jan Vesely <jano.vesely@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>