mba6x_bl: Retry writing backlight on resume/init

On resume we might not have ACPI fully up and running, or other drivers
might be playing with the SMBUS. This is a quick hack around failing
writes on resume. Usually we need 4 retries but try up to 10 times just
to be sure we bring everything back up.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>