ASoC: SOF: loader: Adjust validation condition for fw_offset

ASoC: SOF: loader: Adjust validation condition for fw_offset

Condition in if statement should be same as trace message
and FW size equal to zero (after skipping offset) should
be marked as invalid.

Signed-off-by: Karol Trzcinski <karolx.trzcinski@linux.intel.com>
Signed-off-by: Kai Vehmanen <kai.vehmanen@linux.intel.com>
Reviewed-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>
Reviewed-by: Ranjani Sridharan <ranjani.sridharan@linux.intel.com>
Link: https://lore.kernel.org/r/20200520165911.21696-2-kai.vehmanen@linux.intel.com
Signed-off-by: Mark Brown <broonie@kernel.org>