net: qualcomm: rmnet: Allow partial updates of IFLA_FLAGS

The idiomatic way to handle the changelink flags/mask pair seems to be
allow partial updates of the driver's link flags. In contrast the rmnet
driver masks the incoming flags and then use that as the new flags.

Change the rmnet driver to follow the common scheme, before the
introduction of IFLA_RMNET_FLAGS handling in iproute2 et al.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Alex Elder <elder@linaro.org>
Reviewed-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>