hw/net/lan9118: Signal TSFL_INT flag when TX FIFO reaches specified level

The LAN9118 allows the guest to specify a level for both the TX and
RX FIFOs at which an interrupt will be generated.  We implement the
RSFL_INT interrupt for the RX FIFO but are missing the handling of
the equivalent TSFL_INT for the TX FIFO.  Add the missing test to set
the interrupt if the TX FIFO has exceeded the guest-specified level.

This flag is required for Micrium lan911x ethernet driver to work.

Signed-off-by: Lucas Dietrich <ld.adecy@gmail.com>
[PMM: Tweaked commit message and comment]
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>