scsi: esp_scsi: De-duplicate PIO routines

As a temporary measure, the code to implement PIO transfers was
duplicated in zorro_esp and mac_esp. Now that it has stabilized move the
common code into the core driver but don't build it unless needed.

This replaces the inline assembler with more portable writesb() calls.
Optimizing the m68k writesb() implementation is a separate patch.

[mkp: applied by hand]

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Tested-by: Stan Johnson <userm57@yahoo.com>
Tested-by: Michael Schmitz <schmitzmic@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>