usb: f_qdss: Reset debug_inface_enabled flag upon driver unbind

flag debug_inface_enabled is set to 1 by user space when usb
composition is switched to QDSS having debug interface. This
flag is not set back to 0 upon driver unbind. When composition
is switched to QDSS interface which does not have debug
interface, QDSS driver adds one extra interface to bNumInterface.
This results into host showing extra interface with yellow bang.
Also, fix store() API which is assigning string to a bool variable.

Change-Id: I0a3da15baa0eb67d6216eb0422ee5647a16a766d
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>