firmware: Sigma: Skip header during CRC generation

The firmware header is not part of the CRC, so skip it. Otherwise the firmware
will be rejected due to non-matching CRCs.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: stable@kernel.org