can: cc770: Fix stalls on rt-linux, remove redundant IRQ ack

commit f4353daf4905c0099fd25fa742e2ffd4a4bab26a upstream.

This has been reported to cause stalls on rt-linux.

Suggested-by: Richard Weinberger <richard@nod.at>
Tested-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Andri Yngvason <andri.yngvason@marel.com>
Cc: linux-stable <stable@vger.kernel.org>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>