[ARM] 5090/1: Correct pxafb palette typo error

This patch correct a typo error in pxafb vhich is relevant for 8-bit palette framebuffer configuration.

Signed-off-by: Jrgen Schindele <linux@schindele.name>
Acked-by: Eric Miao <eric.y.miao@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>