fastboot: properly handle unknown partition type

fastboot: properly handle unknown partition type

In getvar_partition_type() we attempt to find a filesystem driver for
the partition (of the list of driver enabled in U-Boot), on failure we
return the error to fastboot and completely bail out of the operation.

However, this should not be a failure, instead we should just default to
"raw". This allows commands like "fastboot format:ext4 userdata" to work
if userdata didn't already have an ext4 partition table (or if FS_EXT4
is disabled in U-Boot), as failing to determine the current partition
type is not an error in this case.

Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Link: https://lore.kernel.org/r/20241113050607.1850472-1-caleb.connolly@linaro.org
Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>