[SCSI] Fix dependency problems in SCSI drivers

We have several drivers that don't list SCSI as a dependency in
Kconfig.  That leads to them potentially being selected as Y even if
SCSI is M (which will produce a build failure).  Fix this by making
the if SCSI_LOWLEVEL that goes around all the drivers a tristate
forcing them all automatically to inherit the value of SCSI.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>