USB: configfs: Don't send DISCONNECT uevent during unbind

Windows XP host takes upto 6 seconds to enumerate RNDIS composition when
RNDIS (tethering) is enabled while in MTP/PTP composition. This results in
Userpsace Tethering enable timeout (1 second) causing RNDIS enumeration
failure and falls back to default composition. Workaround the issue by not
sending disconnect event to userspace on composition switch (MTP/PTP to
RNDIS). Send the disconnect event from USB bus reset so that userspace
enable timer starts from bus reset.

Change-Id: I2d1fcaa0704e369204fbba4eceb8ba9b1c525b41
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>