usb: misc: ipc_bridge: Fix memory leak

Memory for the dev->readbuf is allocated by ipc_bridge_probe function.
But it needs to be explicitly freed by the wrapper driver. Call kfree
on it in the ipc bridge disconnection function.

CRs-Fixed: 639279
Change-Id: I11244abcf7ddd21bf1e7690d070e61c506b14be1
Signed-off-by: Guang Jian Zeng <gzeng@codeaurora.org>