The state field is currently only used to make sure
only the last of the tx and rx dma callbacks issue
an spi_finalize_current_transfer.
Rather than using a spinlock we can get away
with just turning the state field into an atomic_t.
Signed-off-by: Emil Renner Berthing <kernel@esmil.dk> Tested-by: Heiko Stuebner <heiko@sntech.de> Signed-off-by: Mark Brown <broonie@kernel.org>