asus-wireless: Toggle airplane mode LED

In the ASHS device we have the HSWC method, which calls either OWGD or
OWGS, depending on its parameter:

	Device (ASHS)
	{
		Name (_HID, "ATK4002")  // _HID: Hardware ID
		Method (HSWC, 1, Serialized)
		{
			If ((Arg0 < 0x02))
			{
				OWGD (Arg0)
				Return (One)
			}
			If ((Arg0 == 0x02))
			{
				Local0 = OWGS ()
				If (Local0)
				{
					Return (0x05)
				}
				Else
				{
					Return (0x04)
				}
			}
			If ((Arg0 == 0x03))
			{
				Return (0xFF)
			}
			If ((Arg0 == 0x04))
			{
				OWGD (Zero)
				Return (One)
			}
			If ((Arg0 == 0x05))
			{
				OWGD (One)
				Return (One)
			}
			If ((Arg0 == 0x80))
			{
				Return (One)
			}
		}
		Method (_STA, 0, NotSerialized)  // _STA: Status
		{
			If ((MSOS () >= OSW8))
			{
				Return (0x0F)
			}
			Else
			{
				Return (Zero)
			}
		}
	}

On the Asus laptops that do not have an airplane mode LED, OWGD has an
empty implementation and OWGS simply returns 0. On the ones that have an
airplane mode LED these methods have the following implementation:

	Method (OWGD, 1, Serialized)
	{
		SGPL (0x0203000F, Arg0)
		SGPL (0x0203000F, Arg0)
	}

	Method (OWGS, 0, Serialized)
	{
		Store (RGPL (0x0203000F), Local0)
		Return (Local0)
	}

Where OWGD(1) sets the airplane mode LED ON, OWGD(0) set it off, and
OWGS() returns its state.

This commit exposes the airplane mode indicator LED to userspace under
the name asus-wireless::airplane, so it can be driven according to
userspace's policy.

Signed-off-by: João Paulo Rechi Vita <jprvita@endlessm.com>
Reviewed-by: Corentin Chary <corentin.chary@gmail.com>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>