Revert "usb: gadget: Add support for rndis flow control callback"

This reverts commit 56a743ea.

Instead of needing to add a callback to the rndis_params structure,
enable/disable flow control by intercepting the RNDIS_MSG_HALT and
RNDIS_MSG_SET messages directly from gsi_rndis_command_complete().

Change-Id: Id1ecf46d9b4da585be49df4447f073b772b30468
Signed-off-by: Jack Pham <jackp@codeaurora.org>