net: ethernet: ti: cpsw: drop CPSW_HEADROOM define

Since commit 1771afd4 ("net: cpsw: avoid alignment faults by taking
NET_IP_ALIGN into account") the TI CPSW driver was switched to use correct
define CPSW_HEADROOM_NA to avoid alignment faults, but there are two places
left where CPSW_HEADROOM is still used (without causing issues).

Hence, completely drop CPSW_HEADROOM define and use CPSW_HEADROOM_NA
everywhere to avoid further mistakes in code.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>