net: net_tstamp: Add unspec field to hwtstamp_source enumeration

Prepare for future support of saving hwtstamp source in PTP xarray by
introducing HWTSTAMP_SOURCE_UNSPEC to hwtstamp_source enum, setting it
to 0 to match old behavior of no source defined.

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Link: https://patch.msgid.link/20240709-feature_ptp_netnext-v17-5-b5317f50df2a@bootlin.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>