[SCSI] gdth: fix Error: Driver 'gdth' is already registered, aborting...

This message appears on modprobe/rmmod/modprobe of the driver.  It's
caused because if the driver has no instances, it returns an error
from gdth_init, which causes the module to fail to load.
Unfortunately, the module's pci driver is still registered at this
point.

Fix this by making gdth behave like a modern driver and insert even if
it doesn't find any instances (in case of hot plug or software driven
binding).

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