How about something like (with rather arbitrary values)
#define PRIVCMD_DMOP_MAX_NUM_BUFFERS 16
#define PRIVCMD_DMOP_MAX_TOT_BUFFER_SZ 4096
and make them part of the interface (i.e. put them into privcmd.h)?
Given that the values are arbitrary, I think it may be better to make them module params. They can then at least be tweaked if privcmd becomes a problem with later dm_ops.