PM / clk: don't return int on __pm_clk_enable()

Static analysis by cppcheck found an issue that was recently introduced by
commit 471f7707 ("PM / clock_ops: make __pm_clk_enable more generic")
where a return status in ret was not being initialised and garbage
being returned when ce->status >= PCE_STATUS_ERROR.

The fact that ret is not being checked by the caller and that
ret is only used internally __pm_clk_enable() to check if clk_enable()
was OK means we can ignore returning it instead turn
__pm_clk_enable() into function with a void return.

Fixes: 471f7707 ("PM / clock_ops: make __pm_clk_enable more generic")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>