usb: composite: Increase ep0 buffer size to 4KB

Maximum QMI packet size is 4KB. As a result with current
ep0 buffer size of 1KB driver is overstepping the buffer
memory when QMI packet of more than 1KB size is received
from usb host.

Change-Id: Id162ed23dfa855fd7e11d43b32627b577c0c2dd4
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>