[SCSI] aic7xxx/aic79xx: fix module removal path not to panic

In these drivers, scsi_remove_host() is called too late, at the point
it is called, the driver has already shut down too far to accept any
I/O that the shutdown might generate.  Any generated I/O actually
triggers a panic.

Fix this by calling scsi_remove_host() as early as possible and not
calling scsi_host_put() until just before we kfree the ahc_softc.

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