net: mscc: ocelot: split xmit into FDMA and register injection paths

net: mscc: ocelot: split xmit into FDMA and register injection paths

Split ocelot_port_xmit() into two separate functions:
- ocelot_port_xmit_fdma(): handles the FDMA injection path
- ocelot_port_xmit_inj(): handles the register-based injection path

The top-level ocelot_port_xmit() now dispatches to the appropriate
function based on the ocelot_fdma_enabled static key.

This is a pure refactor with no behavioral change. Separating the two
code paths makes each one simpler and prepares for adding proper locking
to the register injection path without affecting the FDMA path.

Signed-off-by: Ziyi Guo <n7l8m4@u.northwestern.edu>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20260208225602.1339325-3-n7l8m4@u.northwestern.edu
Signed-off-by: Jakub Kicinski <kuba@kernel.org>