spi: rockchip: potential NULL dereference on error

We were calling dma_release_channel(rs->dma_tx.ch) when "rs->dma_tx.ch"
is potentially NULL.  There is actually a call to that in the unwind
code at the bottom of the function so we can just re-arrange this a bit
and remove the call.  Also there is no need to set rs->dma_tx.ch to
NULL on this error path.

Fixes: e4c0e06f ('spi: rockchip: fix probe deferral handling')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mark Brown <broonie@kernel.org>