HID: hid-asus: add const to read-only outgoing usb buffer

In the function asus_kbd_set_report the parameter buf is read-only
as it gets copied in a memory portion suitable for USB transfer,
but the parameter is not marked as const: add the missing const and mark
const immutable buffers passed to that function.

Signed-off-by: Denis Benato <benato.denis96@gmail.com>
Signed-off-by: Luke D. Jones <luke@ljones.dev>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>