[ARM] pxa/pcm990: use negative number for an invalid GPIO in camera data

0 is a valid GPIO number, use a negative number to specify, that this camera
doesn't have a GPIO for bus-width switching.

Signed-off-by: Guennadi Liakhovetski <lg@denx.de>
Signed-off-by: Eric Miao <eric.miao@marvell.com>