usb: gadget: Send dedicate uevent for ML client connection

According to mirror link spec, once mirror link client connected,
client will set mirror link command with bRequestType 0x40 and
bRequest 0xF0. In this scenario, USB driver send dedicate uevent
to notify userspace that USB host is a mirror link client.

Change-Id: Ic52f4f7d4e58e879877517fe5775b2f89091fe2a
Signed-off-by: Ziqi Chen <ziqic@codeaurora.org>