[SCSI] Move scsi_print_msg to SPI class

scsi_print_msg() is an SPI-specific concept.  This patch moves it from
constants.c to scsi_transport_spi.c and updates the Kconfig to link in
the SPI class for the drivers which use scsi_print_msg().

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>