[SCSI] scsi_host_lookup: error returns and NULL pointers

This patch cleans up the behavior of scsi_host_lookup().

The original implementation attempted to use the dual role of
either returning a pointer value, or a negative error code.
User's needed to use IS_ERR() to check the result. Additionally,
the IS_ERR() macro never checks for when a NULL pointer was
returned, so a NULL pointer actually passes with a success case.
Note: scsi_host_get(), used by scsi_host_lookup(), can return
a NULL pointer.

Talk about a mudhole for the unitiated to step into....

This patch converts scsi_host_lookup() to return either NULL
or a valid pointer. The consumers were updated for the change.

Signed-off-by: James Smart <james.smart@emulex.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>