USB: gadget: qti: Fix compilation issue for llvm config

Commit 00b09723 ("USB: gadget: qti: Fix NULL check for
usb_pid_string") performs a NULL check for the value which
usb_pid_string is pointing. This check performs a comparison
between char and void* which throws a warning during
compilation. If llvm config is enabled this warning is
treated as a fatal error and compilation fails. Fix this
by adding explicit check for usb_pid_string[0] against zero.

Change-Id: I84b4ed53e400a5702d87f851c724126a3f817d64
Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>