stmmac: platform: Move plat_dat checking earlier

Original code only check/alloc plat_dat for the CONFIG_OF case, this
patch check/alloc it earlier and unconditionally to avoid kernel build
warnings:

drivers/net/ethernet/stmicro/stmmac/stmmac_platform.c:275
stmmac_pltfr_probe() warn: variable dereferenced before check 'plat_dat'

V2: Fix coding style.

Signed-off-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: David S. Miller <davem@davemloft.net>