net: qcom/emac: add software control for pause frame mode

The EMAC has the option of sending only a single pause frame when
flow control is enabled and the RX queue is full.  Although sending
only one pause frame has little value, this would allow admins to
enable automatic flow control without having to worry about the EMAC
flooding nearby switches with pause frames if the kernel hangs.

The option is enabled by using the single-pause-mode private flag.

Signed-off-by: Timur Tabi <timur@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>