stmmac: dwmac-sti: fix st,tx-retime-src check

In case of the st,tx-retime-src is missing from device-tree
(it's an optional field) the driver will invoke the strcasecmp to check
which clock has been selected and this is a bug; the else condition
is needed.

In the dwmac_setup, the "rs" variable, passed to the strcasecmp, was not
initialized and the compiler, depending on the options adopted, could
take it in some different part of the stack generating the hang in such
configuration.

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