USB: gadget: qti: Fix NULL check for usb_pid_string

Currently driver is checking for the usb_pid_string to be
NULL but usb_pid_string will never be NULL since it is a
pointer to a static character array.
Fix this by checking the data rather than the pointer which
usb_pid_string is pointing to.

Change-Id: Ifb3aa7b905201dee7b1c0eb6d4ba902ddcccb135
Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>