usb: f_mtp: Update module parameters only during function bind

Module parameters can be updated through userspace which
can lead to out-of-bound access if unallocated memory is
accessed by the driver. This change prevents out-of-bound
access by updating the module parameters only when
mtp_function_bind is called.

Change-Id: I8c3d9ee64fe194f243dd74b093e72bca1cd05c47
Signed-off-by: Pratham Pratap <prathampratap@codeaurora.org>