net: dsa: lantiq_gswip: optimize regmap_write_bits() statements

net: dsa: lantiq_gswip: optimize regmap_write_bits() statements

Further optimize the previous naive conversion of the *_mask() accessor
functions to regmap_write_bits by manually removing redundant mask
operands.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Acked-by; Hauke Mehrtens <hauke@hauke-m.de>:
Acked-by; Hauke Mehrtens <hauke@hauke-m.de>:
Link: https://patch.msgid.link/fce2f964b22fe3efc234c664b1e50de28dddf512.1761045000.git.daniel@makrotopia.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>