[spi_host] Adjust passthrough not-X assertions
[spi_host] Adjust passthrough not-X assertions

The assertions for the passthrough-related I/Os were overzealous and did
not account for conditions where X is allowable. Adjust them to focus on
SCK and CSB only and have the SCK assertion activate only when CSB is
asserted.

Signed-off-by: Alexander Williams <d0fb8a15e7b6f5e62278a4cf44515038c232186c@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan