usb: gadget: f_serial: Use snprintf() instead of sprintf()

USB serial function driver uses sprintf() to expose port number value to
userspace. sprintf() may result into buffer overrun. Hence use snprintf()
instead of sprintf() which also fixes static analysis tool reported error.

Change-Id: I66532dd9a7bdeb572f0289e393128df4ea4e5f38
Signed-off-by: Mayank Rana <mrana@codeaurora.org>