net: ibm: emac: rgmii: use devm for mutex_init

It seems that since inception, this driver never called mutex_destroy in
_remove. Use devm to handle this automatically.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
Link: https://patch.msgid.link/20241030203727.6039-6-rosenp@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>