net: lan966x: Add define IFH_LEN_BYTES

The total length of IFH(inter frame header) in bytes is calculated as
IFH_LEN * sizeof(u32). Because IFH_LEN describes the length in words
and not in bytes. As the length of IFH in bytes is used quite often,
add a define for this. This is just to simplify the things.

Signed-off-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>