USB: f_mtp: Don't reset string id during function_unbind

Currently MTP driver is resetting string id during unbind, when
cable is disconnected. Due to this, composite driver might run out
of string ids, when it tries to allocate string id during next
composition switch to MTP. This results in composition switch failure
when MTP is selected and eventually causing USB enumeration failure.
Fix this by not resetting string id and allocate it once only.

Change-Id: Ieacdc8dd76bc45638002eb749ff87aa95f496fa3
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>