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.

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