net: macb: Fix check for little-endian system in gmac_configure_dma()

Instead of depending on CONFIG_SYS_LITTLE_ENDIAN, we check at runtime
whether underlying system is little-endian or big-endian. This way
we are not dependent on any U-Boot specific OR compiler specific macro
to check system endianness.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>