The unaligned ioread32() will make us read byte by byte looking for the
vbt. We could just as well have done a ioread8() + a shift and avoid the
extra confusion on how we are looking for "$VBT".

However when using ACPI it's guaranteed the VBT is 4-byte aligned
per spec, so we can probably assume it here as well.

v2: do not try to simplify the loop by eliminating the auxiliary counter
(Jani and Ville)
Signed-off-by: NLucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: NJani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191126225110.8127-4-lucas.demarchi@intel.com