tpm_tis_spi: Prefer async probe

On a Chromebook I'm working on I noticed a big (~1 second) delay
during bootup where nothing was happening.  Right around this big
delay there were messages about the TPM:

[    2.311352] tpm_tis_spi spi0.0: TPM ready IRQ confirmed on attempt 2
[    3.332790] tpm_tis_spi spi0.0: Cr50 firmware version: ...

I put a few printouts in and saw that tpm_tis_spi_init() (specifically
tpm_chip_register() in that function) was taking the lion's share of
this time, though ~115 ms of the time was in cr50_print_fw_version().

Let's make a one-line change to prefer async probe for tpm_tis_spi.
There's no reason we need to block other drivers from probing while we
load.

NOTES:
* It's possible that other hardware runs through the init sequence
  faster than Cr50 and this isn't such a big problem for them.
  However, even if they are faster they are still doing _some_
  transfers over a SPI bus so this should benefit everyone even if to
  a lesser extent.
* It's possible that there are extra delays in the code that could be
  optimized out.  I didn't dig since once I enabled async probe they
  no longer impacted me.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>

drivers/char/tpm/tpm_tis_spi_main.c

@@ -287,6 +287,7 @@ static struct spi_driver tpm_tis_spi_driver = {
 		.pm = &tpm_tis_pm,
 		.of_match_table = of_match_ptr(of_tis_spi_match),
 		.acpi_match_table = ACPI_PTR(acpi_tis_spi_match),
+		.probe_type = PROBE_PREFER_ASYNCHRONOUS,
 	},
 	.probe = tpm_tis_spi_driver_probe,
 	.remove = tpm_tis_spi_remove,