stmmac: remove warning when compile as built-in (V2)

The patch removes the following serie of warnings
when the driver is compiled as built-in:

drivers/net/stmmac/stmmac_main.c: In function stmmac_cmdline_opt:
drivers/net/stmmac/stmmac_main.c:1855:12: warning: ignoring return
value of kstrtoul, declared with attribute warn_unused_result
[snip]

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