USB: gadget: qti: Add support to update product string

Product string is composed in such a way that there is always
a ' ' space character in the string. Since the requirement is
to pass product string via kernel cmdline space has been replaced
by special character '!' in order to maintain the design of
kernel cmdline parameters. This change adds support to update
product string by replacing '!' by ' '.

Change-Id: I839b249ffb198b6d61633ef3909c01ed986afbe7
Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>