stmmac: intel: remove unused 'has_crossts' flag

The 'has_crossts' flag was not used anywhere in the stmmac driver,
removing it from both header file and dwmac-intel driver.

Signed-off-by: Wong Vee Khee <veekhee@apple.com>
Reviewed-by: Kurt Kanzenbach <kurt@linutronix.de>
Link: https://lore.kernel.org/r/20220817064324.10025-1-veekhee@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>