net: mdio: Fix ENOMEM return value in BCM6368 mux bus controller

Error values inside the probe function must be < 0. The ENOMEM return
value has the wrong sign: it is positive instead of negative.
Add a minus sign.

Fixes: e2397567 ("net: mdio: Add BCM6368 MDIO mux bus controller")
Signed-off-by: Niels Dossche <dossche.niels@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20220428211931.8130-1-dossche.niels@gmail.com


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