HID: use debugfs for report dumping descriptor

It is a little bit inconvenient for people who have some non-standard
HID hardware (usually violating the HID specification) to have to
recompile kernel with CONFIG_HID_DEBUG to be able to see kernel's perspective
of the HID report descriptor and observe the parsed events. Plus the messages
are then mixed up inconveniently with the rest of the dmesg stuff.

This patch implements /sys/kernel/debug/hid/<device>/rdesc file, which
represents the kernel's view of report descriptor (both the raw report
descriptor data and parsed contents).

With all the device-specific debug data being available through debugfs, there
is no need for keeping CONFIG_HID_DEBUG, as the 'debug' parameter to the
hid module will now only output only driver-specific debugging options, which has
absolutely minimal memory footprint, just a few error messages and one global
flag (hid_debug).

We use the current set of output formatting functions. The ones that need to be
used both for one-sho...