net: stmmac: increase TX coalesce timer to 5ms

Commit 8fce3331 ("net: stmmac: Rework coalesce timer and fix
multi-queue races") decreased the TX coalesce timer from 40ms to 1ms.

This caused some performance regression on some target (regression was
reported at least on ipq806x) in the order of 600mbps dropping from
gigabit handling to only 200mbps.

The problem was identified in the TX timer getting armed too much time.
While this was fixed and improved in another commit, performance can be
improved even further by increasing the timer delay a bit moving from
1ms to 5ms.

The value is a good balance between battery saving by prevending too
much interrupt to be generated and permitting good performance for
internet oriented devices.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>