fujitsu-laptop: Support radio LED

Lifebook E734/E744/E754 has a LED which the manual calls "radio
components indicator".  It should be lit when any radio transmitter is
enabled.  Its state can be read and set using ACPI (FUNC interface,
RFKILL method).

Since the Lifebook E734/E744/E754 only has a button (as compared to a
slider) for enabling/disabling radio transmitters, I believe the LED in
question is meant to indicate whether all radio transmitters are
currently on or off.  However, pressing the radio toggle button does not
automatically change the hardware state of the transmitters: it looks
like this machine relies on soft rfkill.

As for detecting whether the LED is present on a given machine, I had to
resort to educated guesswork.  I assumed this LED is present on all
devices which have a radio toggle button instead of a slider.  My
Lifebook E744 holds 0x01010001 in BTNI.  By comparing the bits and
buttons with those of a Lifebook E8420 (BTNI=0x000F0101, has a slider),
I put my money on bit 24 as the indicator of the radio toggle button
being present.  Furthermore, bit 24 is also clear on the S7020 which
does not have the toggle button or an RF LED.

Figuring out how the LED is controlled was more deterministic as all it
took was decompiling the DSDT and taking a look at method S000 (the
RFKILL method of the FUNC interface).

The LED control method implemented here is unsuitable for use with
"heavy" LED triggers, like phy0rx.  Once blinking frequency achieves a
certain level, the system hangs.

Signed-off-by: Michał Kępień <kernel@kempniu.pl>
[jwoithe: Comment on bit 24 in BTNI, expanded commit msg]
Signed-off-by: Jonathan Woithe <jwoithe@just42.net>
[dvhart: Minor style and commit log adjustments]
Signed-off-by: Darren Hart <dvhart@linux.intel.com>