spi: at91-usart: Remove some dead code

spi: at91-usart: Remove some dead code

dma_request_chan() does not return NULL. It returns a valid pointer or an
error pointer.

So, some dead code can be removed.

The IS_ERR_OR_NULL() in the error handling path are still needed, because
the error handling path is common to the whole function and the
ctlr->dma_xx are NULL when at91_usart_spi_configure_dma() is called.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/84eb08daf85d203b34af9d8d08abf86804211413.1694961365.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Mark Brown <broonie@kernel.org>