net: renesas: rswitch: rename rswitch.c to rswitch_main.c

net: renesas: rswitch: rename rswitch.c to rswitch_main.c

Adding new functionality to the driver. Therefore splitting into multiple
c files to keep them manageable. New functionality will be added to
separate files.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Michael Dege <michael.dege@renesas.com>
Link: https://patch.msgid.link/20250901-add_l2_switching-v5-1-5f13e46860d5@renesas.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>