net: pcs: xpcs: mask readl() return value to 16 bits

readl() returns 32-bit value but Clause 22/45 registers are 16-bit wide.
Masking with 0xFFFF avoids using garbage upper bits.

Signed-off-by: Jack Ping CHNG <jchng@maxlinear.com>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20250716030349.3796806-1-jchng@maxlinear.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>