r8169: improve rtl_tx

We can simplify the for() condition and eliminate variable tx_left.
The change also considers that tp->cur_tx may be incremented by a
racing rtl8169_start_xmit().
In addition replace the write to tp->dirty_tx and the following
smp_mb() with an equivalent call to smp_store_mb(). This implicitly
adds a WRITE_ONCE() to the write.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://lore.kernel.org/r/c2e19e5e-3d3f-d663-af32-13c3374f5def@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>