usb: gadget: Move req->length check from function drivers to composite

req->legth is set as 0 before calling individual function driver's
setup routines. Hence, check for the buffer length in composite_setup
only.
Also, change max buffer size to 4k to accomodate our descriptors.

Change-Id: Ic96408210e7e5576928a7147d2d8a57fa620d287
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>