usb: gadget: ci: Avoid null pointer dereference

The ci_req->hw_buf can be NULL, test whether it is and if so,
avoid accessing it. Else, the system may crash.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Peter Chen <peter.chen@nxp.com>
Cc: Li Jun <jun.li@nxp.com>
Cc: Peng Fan <peng.fan@nxp.com>