stmmac: intel: Fix clock handling on error and remove paths

clk_prepare_enable() might fail, we have to check its returned value.
Besides that we have to call clk_disable_unprepare() on the error and
remove paths. Do above in the dwmac-intel driver.

While at it, remove leftover in stmmac_pci and remove unneeded condition
for NULL-aware clk_unregister_fixed_rate() call.

Fixes: 58da0cfa ("net: stmmac: create dwmac-intel.c to contain all Intel platform")
Cc: Voon Weifeng <weifeng.voon@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>