[SCSI] ncr53c8xx: Call scsi_host_put in release

Since ncr53c8xx_attach() calls scsi_host_put(), make ncr53c8xx_release()
call scsi_host_put() too, for symmetry.  Both callers already expect
it to put the host for them, so that works out nicely.  While the zalon
driver does 'use' the host pointer afterwards, it only compares it for
equality and doesn't dereference it, so that's safe.

While I'm at it, get rid of pointless checks for NULL, use shost_priv()
and change ncr53c8xx_release to return void.

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>