usb: android: Fix buffer overflow due to incorrect snprintf length

The size_t parameter used with snprintf for lun name refers
to a different buffer while the actual buffer is 5 bytes long.
Fix this by passing the correct size_t parameter.

Change-Id: Ic8a827b82159fa2903e8047841b0ad35733b0c00
Signed-off-by: Devdutt Patnaik <dpatnaik@codeaurora.org>