bcm63xx_enet: correct clock usage

commit 9c86b846 upstream.

Check the return code of prepare_enable and change one last instance of
enable only to prepare_enable. Also properly disable and release the
clock in error paths and on remove for enetsw.

Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>