bebob: add set_dig_iface() to change digital interface

In previous commit, there is a misunderstanding of set_clock_params(). Two of
these params are related to digital format (ADAT or S/PDIF). I add set dig
iface() to switch coaxial/optical for S/PDIF mode.

For control interface, convination of digital format and interface is given as
option, referring to Fireworks.

Additionally this commit unify similar functions between FW1814 and ProjectMix.