firmware-utils/tplink-safeloader: soft-version magic is data length

firmware-utils/tplink-safeloader: soft-version magic is data length

The soft-version partition actually contains a header and trailing data:
* header: {data length, [zero]}
* data: {version, bcd encoded date, revision}

The data length is currently treated as a magic number, but should
contain the length of the partition data.

This header is also present the following partitions (non-exhaustive):
* string-based soft-version
* support-list

Signed-off-by: Sander Vanheule <sander@svanheule.net>