staging: unisys: visorbus: Remove POSTCODE_SEVERITY levels

Use the already defined DIAG_SEVERITY values directly instead of special
postcode macro names.

Signed-off-by: Bryan Thompson <bryan.thompson@unisys.com>
Signed-off-by: David Kershner <david.kershner@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>