mfd: max8925: fix dt code for backlight

The device-tree enablement for max8925 has several problems, but besides
the bindings being wrong (and not having seen review) there's also some
bad coding practices on how to fill in the platform_data from device tree.

I came across this since it causes a warning when compiling
mmp2_defconfig, and instead of doing the minimal fix to silence the
warning, I restructured the code a bit.

This silences the warning:
drivers/video/backlight/max8925_bl.c: In function 'max8925_backlight_probe':
drivers/video/backlight/max8925_bl.c:177:3: warning: statement with no effect [-Wunused-value]

Note that the bindings themselves need to be revisited too, but that will
affect more than just the backlight driver and is best done separately;
this just fixes the bad code for the backlight driver.

Acked-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Olof Johansson <olof@lixom.net>