net: bcmgenet: Use devm_clk_get_optional() to get the clocks

Conversion to devm_clk_get_optional() makes it explicit that clocks are
optional. This change allows to handle deferred probe in case clocks are
defined, but not yet probed. Due to above changes bail out in error case.

While here, check potential error when enable main clock.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>