mcdx has different IRQ numbers for all drives; irq handler needs a
pointer to structure describing the drive in question.  However, instead
of passing such pointer at request_irq() time and having it passed to
mcdx_intr() for free, mcdx.c sticks the pointer in question into an
array indexed by IRQ number and has mcdx_intr() go look it up there. 

Cleaned up, array killed.