usb: simplify request handlers code

Now that Standard, Class and Vendor request handlers are using the same
buffer, we do not need to keep and array installed request data pointer.
This simplifies the code.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>