net: dsa: realtek: rtl8365mb: prepare for multiple source files

net: dsa: realtek: rtl8365mb: prepare for multiple source files

Rename rtl8365mb.c to rtl8365mb_main.c in preparation for subsequent
commits which add additional source files to the driver.

The trailing backslash in the Makefile is deliberate. It allows for new
files to be added without clobbering git history.

Signed-off-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Reviewed-by: Linus Walleij <linusw@kernel.org>
Reviewed-by: Mieczyslaw Nalewaj <namiltd@yahoo.com>
Co-developed-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Link: https://patch.msgid.link/20260606-realtek_forward-v13-4-b9e409687cbe@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>