SPI: Fix command-response behavior

Fix SPI flash ops regressions after commit:
   c2973d1 spi: Get rid of SPI_ATOMIC_SEQUENCING

When spi_flash_cmd() is called with argument response==NULL,
only send out command without reading back the response.

Change-Id: I28a94f208b4a1983d45d69d46db41391e267891d
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/18082
Tested-by: build bot (Jenkins)
Reviewed-by: Timothy Pearson <tpearson@raptorengineering.com>
Tested-by: Raptor Engineering Automated Test Stand <noreply@raptorengineeringinc.com>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
1 file changed