gma500: fix build warning

 CC [M]  drivers/gpu/drm/i915/i915_gem_evict.o
/ssd/git/drm-core-next/drivers/gpu/drm/gma500/psb_device.c: In function ‘psb_chip_errata’:
/ssd/git/drm-core-next/drivers/gpu/drm/gma500/psb_device.c:360:1: warning: no return statement in function returning non-void [-Wreturn-type]
/ssd/git/drm-core-next/drivers/gpu/drm/gma500/psb_device.c: At top level:
/ssd/git/drm-core-next/drivers/gpu/drm/gma500/psb_device.c:379:2: warning: initialization from incompatible pointer type [enabled by default]
/ssd/git/drm-core-next/drivers/gpu/drm/gma500/psb_device.c:379:2: warning: (near initialization for ‘psb_chip_ops.errata’) [enabled by default]

Signed-off-by: Dave Airlie <airlied@redhat.com>