snd_hdspe(4): Per device sysctl for sample rate.

Some hardware setups require a specific sample rate due to devices being
connected to digital ports (AES, S/PDIF, ADAT). Add a per device sysctl
"sample_rate" to let the user override sample rate requests from the pcm
infrastructure, when needed.

Differential Revision: https://reviews.freebsd.org/D43659