[ARM] pata_icside: fix the FIXMEs

Alan Cox suggested that the solution to the FIXMEs in pata_icside is
to use a private postreset method to detect the lack of devices on a
port, and in such a case, disable the interrupt for the port.

This patch implements such a method, and removes the hard coded
disable of port 0.  Tested as working.

Acked-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>