net: macb: Clear SRTSM outside PTPv2 receive filters

A PTPv2 receive filter sets NCR.SRTSM, but switching to NONE, ALL or a
PTPv1 filter leaves it set. As a result, selecting the same filter can
produce a different register setting depending on whether PTPv2 was
used before.

Update SRTSM whenever the RX filter changes, setting it only for PTPv2.
Switching away from PTPv2 then clears the old setting, so the bit follows
the currently selected filter.

Reviewed-by: Nicolai Buchwitz <nb@tipi-net.de>
Reviewed-by: Théo Lebrun <theo.lebrun@bootlin.com>
Tested-by: Nicolai Buchwitz <nb@tipi-net.de>
Signed-off-by: Kim Wooseok <5mghybrid@khu.ac.kr>
Link: https://patch.msgid.link/20260923-macb-hwtstamp-rework-v2-3-75c909b2676d@khu.ac.kr
Signed-off-by: Jakub Kicinski <kuba@kernel.org>