hid-input: add support for HID devices reporting Battery Strength

I've sent an email earlier asking for help with a GetFeature code, and now I
have a second patch on top of Jeremy's to provide the battery functionality
for devices that support reporting it.

If I understood correctly when talking to Jeremy he said his device
never actually reported the status as an input event (sorry if I didn't
understand it correctly), and after reading HID specs I believe it's
really because it was meant to be probed, I have an Apple Keyboard and
Magic Trackpad both bluetooth batteries operated, so using PacketLogger
I saw that Mac OSX always ask the battery status using the so called
GetFeature.

What my patch does is basically:
- store the report id that matches the battery_strength
- setup the battery if 0x6.0x20 is found, even if that is reported as a feature
  (as it was meant to be but only the MagicTrackpad does)
- when upower or someone access /sys/class/power_supply/hid-*/capacity it
  will probe the device and return it's status.

It works great for both devices, but I have two concerns:
- the report_features function has a duplicated code
- it would be nice if it was possible for specific drivers to provide their own
  probe as there might be some strange devices... (but maybe it's
already possible)

I've talked to the upower dev and he fixed it to be able to show the
right percentage.

Here how the uevent file (in /sys/class/power_supply/hid-*/) looks like:
POWER_SUPPLY_NAME=hid-00:22:41:D9:18:E7-battery
POWER_SUPPLY_PRESENT=1
POWER_SUPPLY_ONLINE=1
POWER_SUPPLY_CAPACITY=66
POWER_SUPPLY_MODEL_NAME=MacAdmin’s keyboard
POWER_SUPPLY_STATUS=Discharging

POWER_SUPPLY_NAME=hid-70:CD:60:F5:FF:3F-battery
POWER_SUPPLY_PRESENT=1
POWER_SUPPLY_ONLINE=1
POWER_SUPPLY_CAPACITY=62
POWER_SUPPLY_MODEL_NAME=nexx’s Trackpad
POWER_SUPPLY_STATUS=Discharging

Signed-off-by: Daniel Nicoletti <dantti12@gmail.com>