spi: spi-topcliff-pch: Fix potential double free in pch_spi_process_messages()

spi: spi-topcliff-pch: Fix potential double free in pch_spi_process_messages()

pch_spi_set_tx() frees data->pkt_tx_buff on failure of kzalloc() for
data->pkt_rx_buff, but its caller, pch_spi_process_messages(), will
free data->pkt_tx_buff again. Set data->pkt_tx_buff to NULL after
kfree() to avoid double free.

Signed-off-by: Jay Fang <f.fangjian@huawei.com>
Link: https://lore.kernel.org/r/1620284888-65215-1-git-send-email-f.fangjian@huawei.com
Signed-off-by: Mark Brown <broonie@kernel.org>