usb: gadget: Fix race condition in enable\disable Rmnet device

This fixes race condition in fast disable and re-enable Rmnet device.
Now the mechanism is similar to the connect\disconnect mechanism
in u_bam_data.

CRs-Fixed: 524776
Change-Id: Idfbb5904be05f66ea404c1a806e4d224ecd553df
Signed-off-by: Nadine Toledano <nadinet@codeaurora.org>