m68k/mac: cleanup mac_irq_pending

mac_irq_pending() has only one caller (mac_esp.c). Nothing tests for Baboon, PSC or OSS pending interrupts. Until that need arises, let's keep it simple and remove all the unused abstraction. Replace it with a routine to check for SCSI DRQ.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>