net: ethernet: fs_enet: simplify clock handling with devm accessors

devm_clock_get_enabled() can be used to simplify clock handling for the
PER register clock.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>