spi: at91-usart: Use PTR_ERR_OR_ZERO() to simplify code

Return PTR_ERR_OR_ZERO() instead of return 0 or PTR_ERR() to
simplify code.

Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Link: https://lore.kernel.org/r/20230822124643.987079-1-ruanjinjie@huawei.com


Signed-off-by: Mark Brown <broonie@kernel.org>