maple: make maple_bus_type static and const

maple: make maple_bus_type static and const

There is no need to export maple_bus_type as no one uses it outside of
maple.c, so make it static, AND make it const as it can be read-only as
no one modifies it.

Cc: Yoshinori Sato <ysato@users.sourceforge.jp>
Cc: Rich Felker <dalias@libc.org>
Cc: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Cc: <linux-sh@vger.kernel.org>
Link: https://lore.kernel.org/r/2023121918-rejoicing-frostlike-d976@gregkh
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>