net: phy: clean up mmd_phy_indirect()

Make mmd_phy_indirect() use the same terminology as the rest of the
code, making clear what each address is - phy address, devad, and
register number.

While here, remove the "inline" from this static function, leaving
it to the compiler to decide whether to inline this function, and
get rid of unnecessary parens.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>