ALSA: aloop - Close races at restarting the stream

There are small races opened in the check of running bit and the timer
lock.  Instead of adding yet more flag, just protect the whole racy
codes with the existing cable->lock.  As a bonus, we can get rid of
timer_lock now.

Reported-and-tested-by: Omair Mohammed Abdullah <omair.m.abdullah@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>