sound: Retire SD_F_PRIO_*

The SD_F_* flags are supposed to be softc flags, but SD_F_PRIO_RD and
SD_F_PRIO_WR are just generic flags and are only used with
dsp_lock_chans() and dsp_unlock_chans(). Since we already have the
DSP_F_READ() and DSP_F_WRITE() macros, we can re-use them instead.

I am aware the FREAD and FWRITE flags are meant to be used with open(),
but I think their use here is clear enough to not cause confusion.

No functional change intended.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D49978