mfd: ocelot-spi: Use spi_sync_transfer()

Use spi_sync_transfer() instead of hand-writing it.

It is less verbose.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Colin Foster <colin.foster@in-advantage.com>
Tested-by: Colin Foster <colin.foster@in-advantage.com>
Link: https://lore.kernel.org/r/7af920eb686b719cb7eb39c832e3ad414e0e1e1a.1712258667.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Lee Jones <lee@kernel.org>