realtek: add Linux Kernel 5.15 as testing version

realtek: add Linux Kernel 5.15 as testing version

Add Linux Kernel 5.15 support for testing.

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
[APRESIA ApresiaLightGS120GT-SS, Panasonic Switch-M24eG PN28240K, Switch-M48eG PN28480K]
Tested-by: INAGAKI Hiroshi <musashino.open@gmail.com>
TP-Link TL-SG2008P
Tested-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
[D-Link DGS-1210-20, DGS-1210-52, Zyxel XGS1010-12]
Tested-by: Markus Stockhausen <markus.stockhausen@gmx.de>
[Zyxel XGS1250-12]
Tested-by: Lucian Cristian <lucian.cristian@gmail.com> # Zyxel
[HPE 1920-8G, 1920-48G]
Tested-by: Jan Hoffmann <jan@3e8.eu>