dsa: qca8k: Use nested lock to avoid splat

qca8k_phy_eth_command() is used to probe the child MDIO bus while the
parent MDIO is locked. This causes lockdep splat, reporting a possible
deadlock. It is not an actually deadlock, because different locks are
used. By making use of mutex_lock_nested() we can avoid this false
positive.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20241110175955.3053664-1-andrew@lunn.ch


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