net: stmmac: gmac5+: fix potential integer overflow on 32 bit multiply

The multiplication of cfg->ctr[1] by 1000000000 is performed using a
32 bit multiplication (since cfg->ctr[1] is a u32) and this can lead
to a potential overflow. Fix this by making the constant a ULL to
ensure a 64 bit multiply occurs.

Fixes: 504723af ("net: stmmac: Add basic EST support for GMAC5+")
Addresses-Coverity: ("Unintentional integer overflow")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>