[SCSI] m68k: mac_esp asm fix

Fix asm constraints and arguments so as not to transfer an odd byte when
there may be more words to transfer. The bug would probably also cause
exceptions sometimes by transferring one too many bytes.

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