usb: gadget: gserial: check if console kthread exists

Check for bad pointer that may result because of kthread_create failure.
This check is needed since the gserial setup callback function
(gs_console_setup()) is only freeing the info->con_buf in case of
kthread_create failure which will result into bad info->console_thread
pointer.
Without checking info->console_thread pointer validity in the
gserial_console_exit() function, before calling kthread_stop(), the
rmmod will generate Kernel Oops.

Signed-off-by: Bogdan Mirea <Bogdan-Stefan_mirea@mentor.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>