pwm_backlight: add flags to pwmbus, fix percent calculation

[oskar.holmlund_ohdata.se]

Summary

Add call to PWMBUS_CHANNEL_SET_FLAGS in attach(), dont expect the flags to change in pwm_backlight_update_status().

Fix percent calculation in call to pwm_backlight_find_level_per_percent() in the attach() function.

Fix linelength.