USB: composite: Use snprintf instead of sprintf

composite driver is using sprintf() function to expose value to
userspace. This sprintf() is banned function and instead required
to use snprintf() function. Hence replace sprintf() with snprintf()
function to fix static analysis tool error.

Change-Id: I85f38fe3f2121f6a0cb76b7178a3e699ba1dc7f2
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>