PM / clock_ops: report clock errors from clk_enable()

If clk_enable() fails, then print a message so that the user can see
what is happening instead of silently failing to enable the clock.

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Reviewed-by: Ian Molton <ian.molton@codethink.co.uk>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>