acpi_asus_wmi(4): Improve keyboard backlight support.

• Fix maximal keyboard backlight level, Although Linux source code comment says that backlight level is encoded in 3 bits of data, actual code limits maximum level to 3.
• Add backlight(9) support for keyboard
• Turn off/on keyboard backlight on suspend/resume

Sponsored by: Future Crew LLC
MFC after: 1 month
Reviewed by: mav
Differential Revision: https://reviews.freebsd.org/D48983