drivers/spi/tpm: de-assert chip select on transaction error

In the case of start_transaction() failing the chip select is never
deasserted. Correct that by deasserting the chip select when
start_transaction() fails.

BUG=b:36598499

Change-Id: I2c5200085eb357259edab39c1a0fa7b1d81ba7b2
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/19056
Tested-by: build bot (Jenkins)
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/drivers/spi/tpm/tpm.c

@@ -171,6 +171,7 @@ static int start_transaction(int read_write, size_t bytes, unsigned addr)
 	do {
 		if (stopwatch_expired(&sw)) {
 			printk(BIOS_ERR, "TPM flow control failure\n");
+			tpm_if.cs_deassert(&tpm_if.slave);
 			return 0;
 		}
 		tpm_if.xfer(&tpm_if.slave, NULL, 0, &byte, 1);