ALSA: hda-intel - fix wallclk variable update and condition

This patch fixes thinko introduced in "last minutes" before commiting
of the last wallclk patch.

It also fixes the condition checking if the first period after last
wallclk update is processed. There is a little rounding error in
period_wallclk.

Signed-off-by: Jaroslav Kysela <perex@perex.cz>