usb: gadget: OS String support

usb: gadget: OS String support

There is a custom (non-USB IF) extension to the USB standard:

http://msdn.microsoft.com/library/windows/hardware/gg463182

They grant permission to use the specification - there is
"Microsoft OS Descriptor Specification License Agreement"
under the link mentioned above, and its Section 2 "Grant
of License", letter (b) reads:

"Patent license. Microsoft hereby grants to You a nonexclusive,
royalty-free, nontransferable, worldwide license under Microsoft’s
patents embodied solely within the Specification and that are owned
or licensable by Microsoft to make, use, import, offer to sell,
sell and distribute directly or indirectly to Your Licensees Your
Implementation. You may sublicense this patent license to Your
Licensees under the same terms and conditions."

The said extension is maintained by Microsoft for Microsoft.

Yet it is fairly common for various devices to use it, and a
popular proprietary operating system expects devices to provide
"OS descriptors", so Linux-based USB gadgets whishing to be able
to talk to a variety of operating systems should be able to provide
the "OS descriptors".

This patch adds optional support for gadgets whishing to expose
the so called "OS String" under index 0xEE of language 0.
The contents of the string is generated based on the qw_sign
array and b_vendor_code.

Interested gadgets need to set the cdev->use_os_string flag,
fill cdev->qw_sign with appropriate values and fill cdev->b_vendor_code
with a value of their choice.

This patch does not however implement responding to any vendor-specific
USB requests.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>