USB: gadget: rndis: Allow reading of 'rx_trigger' attribute value

rx_trigger attribute is used by userspace to kickstart RX
once end to end data path is setup on USB connection.
Currently this attribute is write only. Make it readable
so that userspace can read it back during fast USB connect
disconnect scenarios which results in net core missing to
send NETLINK DISCONNECT events. Userspace can read this back
on CONNECT to figure out any missed disconnect events.

Change-Id: Ief1d93b1e29103336b672cf0e8abcfe881aaa617
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>