[SCSI] gdth: fix timer handling

The global timer handling is problematic in that if someone unbinds a
PCI gdth instance, the BUG_ON() in the timer will cause a panic.

Fix this by making the timer start and stop depending on whether there
are instances present.  This should also permit binding and unbinding
to work.

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