Input: xpad - correctly handle concurrent LED and FF requests

Input: xpad - correctly handle concurrent LED and FF requests

Track the status of the irq_out URB to prevent submission iof new requests
while current one is active. Failure to do so results in the "URB submitted
while active" warning/stack trace.

Store pending brightness and FF effect in the driver structure and replace
it with the latest requests until the device is ready to process next
request. Alternate serving LED vs FF requests to make sure one does not
starve another. See [1] for discussion. Inspired by patch of Sarah Bessmer
[2].

[1]: http://www.spinics.net/lists/linux-input/msg40708.html
[2]: http://www.spinics.net/lists/linux-input/msg31450.html

Signed-off-by: Pavel Rojtberg <rojtberg@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>