net: ravb: Break out Rx hardware timestamping

net: ravb: Break out Rx hardware timestamping

Prepare for moving away from device specific bit-fields to track how to
do hardware Rx timestamping to using net common enums by breaking out
the timestamping to a helper function. This is done to create cleaner
code and prepare for easier changes improving the hardware timestapming.

There is no functional change.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://patch.msgid.link/20251104222420.882731-7-niklas.soderlund+renesas@ragnatech.se
Signed-off-by: Jakub Kicinski <kuba@kernel.org>