net: update docbook comment for __mdiobus_register()

Update the docbook comment for __mdiobus_register() to include the new
module owner argument.  This resolves a warning found by the 0-day
builder.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>