greybus: audio-pcm: fix use of variable unitialized

Compiling with clang shows that period_elapsed will be used as a branch
conditional unitialized whenever snd_dev->transfer_done <
runtime->period_size. Since stack can grow up/down as we proceed though
the call stack this should be fixed. This patch fixes by explicitly
initalizing period_elapsed to zero.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reviewed-by: Alex Elder <elder@linaro.org>
Reviewed-by: Mark Greer <mgreer@animalcreek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>