spi: img-spfi: fix potential double release

spi: img-spfi: fix potential double release

The channels spfi->tx_ch and spfi->rx_ch are not set to NULL after they
are released. As a result, they will be released again, either on the
error handling branch in the same function or in the corresponding
remove function, i.e. img_spfi_remove(). This patch fixes the bug by
setting the two members to NULL.

Signed-off-by: Pan Bian <bianpan2016@163.com>
Link: https://lore.kernel.org/r/1573007769-20131-1-git-send-email-bianpan2016@163.com
Signed-off-by: Mark Brown <broonie@kernel.org>