[SCSI] advansys: Remove asc_board_t typedef and ASC_BOARDP macro

asc_board_t was simply a typedef for struct asc_board.  ASC_BOARDP()
can be replaced by shost_priv() except in the ASC_STATS* macros which
rely on the cast; add an explicit cast there.
Signed-off-by: Matthew Wilcox <willy@linux.intel.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>