HID: semitek: new driver for GK6X series keyboards

A number of USB keyboards, using the Semitek firmware, are capable of
handling arbitrary N-key rollover, but due to a buggy report
descriptor, keys beyond the sixth cannot be detected by the generic
HID driver.

There are numerous hardware variants sold by several vendors, mostly
using generic names like "GK61" for the 61-key version.  These
keyboards are sometimes known collectively as the "GK6X" series.

The keyboard has three USB interfaces.  Interface 0 uses the standard
HID boot protocol, limited to eight modifier keys and six normal keys;
interface 2 uses a custom report format that permits any number of
keys.  If more than six keys are pressed simultaneously, the first six
are reported via interface 0 while subsequent keys are reported via
interface 2.

(Interface 1 uses a custom protocol for reprogramming the keyboard;
this can be controlled through userspace tools and is not of concern
for the present driver.)

The report descriptor for interface 2, however, is incorrect (for
report ID 0x04, the input field is marked as "array" rather than
"variable".)  The descriptor appears to be correct in other respects,
so we simply replace the incorrect byte before parsing the descriptor.

Signed-off-by: Benjamin Moody <bmoody@member.fsf.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>