net: broadcom: bcm4908_enet: drop "inline" from C functions

It seems preferred to let compiler optimize code if applicable.
While at it drop unused enet_umac_maskset().

Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>