usb: gadget: f_hid: Fix unbind issues

Implement kref tracking for f_hid structure so that it
does not get freed while user space has reference to the
/dev/hidgX nodes.

When the hidg is unbound, unblock poll/read/write. The expectation
is that user space close and open the device nodes created upon the
next bind.

Change-Id: I19701189ed4d564f88052ef0f079880c3e744d47
Signed-off-by: Pavankumar Kondeti <quic_pkondeti@quicinc.com>