stmmac: fix timer setup when use dual mac Kconfig

The driver erroneously sets the tmrate to zero when the
TMU initialisation fails. This actually generates problems
while using the dual GMAC configuration.

With this patch, enabling both the dual gmac and the timer
optimisation, the first interface opened will use the tmu
channel 2, the second one won't be able to use the timer but
will continue to work without mitigating the interrupts by
using the external timer (i.e. TMU channel 2).

Signed-off-by: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>