scsi: atari_scsi: Use m68k_realnum_memory for FastRAM test

m68k_num_memory is unsuitable to test for the presence of FastRAM on
CT60 if the kernel is located in FastRAM: in arch/m68k/mm/motorola.c the
ST-RAM chunk is skipped and m68k_num_memory is decremented in this
case. m68k_realnum_memory still contains the actual number of RAM chunks
so use that.

Signed-off-by: Michael Schmitz <schmitzmic@gmail.com>
Tested-by: Christian T. Steigies <cts@debian.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>