net: stmmac: Fix missing { } around two statements in an if statement

There are missing { } around a block of code on an if statement. Fix this
by adding them in.

Addresses-Coverity: ("Nesting level does not match indentation")
Fixes: 46682cb8 ("net: stmmac: enable Intel mGbE 2.5Gbps link speed")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>