net: stmmac: xgmac: fix l4 filter port overwrite on register update

net: stmmac: xgmac: fix l4 filter port overwrite on register update

The XGMAC_L4_ADDR register holds both source and destination port
match values. The current implementation overwrites the entire register
when configuring either port, so setting one silently erases the other.

Fix this by reading the register first, then masking and updating only
the relevant field before writing back.

Fixes: 425eabddaf0f ("net: stmmac: Implement L3/L4 Filters using TC Flower")
Signed-off-by: Rohan G Thomas <rohan.g.thomas@altera.com>
Signed-off-by: Nazim Amirul <muhammad.nazim.amirul.nazle.asmade@altera.com>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20260714023716.29865-3-muhammad.nazim.amirul.nazle.asmade@altera.com
Reviewed-by: Jakub Raczynski <j.raczynski@samsung.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>