net: bcmgenet: return EOPNOTSUPP for unknown ioctl commands

This commit changes the ioctl handling behavior to return the
EOPNOTSUPP error code instead of the EINVAL error code when an
unknown ioctl command value is detected.

It also removes some redundant parsing of the ioctl command value
and allows the SIOCSHWTSTAMP value to be handled.

Signed-off-by: Doug Berger <opendmb@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>