snd_hdspe(4): Per device sysctl for period.

Let the user choose a period (interrupt cadence in samples), in the
official RME drivers this setting is available as "Buffer Size".
Override the period propagated through blocksize by pcm channel latency
settings (see sound(4)), since these are unreliable and differ between
playback and recording channels.

Differential Revision: https://reviews.freebsd.org/D43527