net: core: Inline netdev_features_size_check()

We do not require this inline function to be used in multiple different
locations, just inline it where it gets used in register_netdevice().

Suggested-by: David Miller <davem@davemloft.net>
Suggested-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>