realtek: pcs: rtl93xx: rename media to attachment

realtek: pcs: rtl93xx: rename media to attachment

"media" doesn't fit well once PHY sits alongside FIBER/DAC_SHORT/
DAC_LONG as a value: a PHY isn't a transmission medium, whereas what
actually matters here is what electrically terminates the SerDes lane
on the other end. Rename the enum, struct field, ops vtable member,
and both variants' config functions accordingly. Calling it 'attachment'
better fits what it actually is, doesn't confuse with the variants and
also fits to the PMA (Physical Medium Attachment) sublayer that lives
below PCS.

Link: https://github.com/openwrt/openwrt/pull/24232
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>