net: qualcomm: rmnet: only set flags when given

Recent userspace application changes affecting device creation have
exposed a race between changing the data format flags and processing
packets. If new devices were created without specifying the flags
argument, the data format for all rmnet devices would be overwritten,
possibly in the middle of processing. Fix this by only changing the data
format when creating a new rmnet device if a new data format is given to
us. Otherwise, the data format will be left alone.

Change-Id: I7bd5508659503bb69bc3b1b5504bdad98c927745
Signed-off-by: Sean Tranchetti <stranche@codeaurora.org>