include: net_if: fix clarification of timestamp callback

The functions of timestamp callback were implemented under
CONFIG_NET_PKT_TIMESTAMP_THREAD, but were clarified under
CONFIG_NET_PKT_TIMESTAMP.

The clarification in header file may misunderstand users,
because enabling CONFIG_NET_PKT_TIMESTAMP is not enough
to use these functions.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>