On i386 compile the back-end with EISA support as well as the EISA
front-end if the dpt(4) module is built along with a kernel that
includes eisa(4) or when compiling it stand-alone (logic based on
the corresponding ISA logic in sys/modules/sound/sound/Makefile).
As as side-effect this fixes the stand-alone build of the dpt(4)
module after dpt.h 1.17, dpt_eisa.c 1.22 and dpt_scsi.c 1.55.

Breakage reported by: n_hibma