[spi_device] Minor lint fix
[spi_device] Minor lint fix

AscentLint complains about a two-state type variable being assigned
to 4 state logic. Solution: make it a 4 state type.

Signed-off-by: Michael Schaffner <a005a2d3342a687ea2d05360c953107acbe83e08@google.com>

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