net: phylink: simplify phylink_parse_fixedlink()

phylink_parse_fixedlink() wants to preserve the pause, asym_pause and
autoneg bits in pl->supported. Rather than reading the bits into
separate bools, zeroing pl->supported, and then setting them if they
were previously set, use a mask and linkmode_and() to achieve the same
result.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/E1t3Fh5-000aQi-Nk@rmk-PC.armlinux.org.uk


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