acer-wmi: fix section mismatch warnings

Fix following warnings:
WARNING: vmlinux.o(.text+0x672615): Section mismatch in reference from the function acer_platform_remove() to the function .exit.text:acer_backlight_exit()
WARNING: vmlinux.o(.devinit.text+0x1e859): Section mismatch in reference from the function acer_platform_probe() to the function .init.text:acer_led_init()
WARNING: vmlinux.o(.devinit.text+0x1e878): Section mismatch in reference from the function acer_platform_probe() to the function .init.text:acer_backlight_init()

Remove __exit annotation from acer_backlight_exit(). We cannot reference
a __exit annotated function from non __exit functions.

acer_led_init() and acer_backlight_init() where both annotated __init but
used from a __devinit function. This would result in an oops should
gcc drop their inlining and the module are hot plugged.

Fix by annotating acer_led_init() and acer_backlight_init() __devinit.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Carlos Corbacho <carlos@strangeworlds.co.uk>
Signed-off-by: Len Brown <len.brown@intel.com>