[ARM] 5077/1: spi: fix list scan success verification in PXA ssp driver

The list search success check in arch/arm/mach-pxa/ssp.c is wrong: for
example, it didn't recognise failure for me when I requested port 0.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@pengutronix.de>
Acked-by: Eric Miao <eric.miao@marvell.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>