Fix PMP grain assertion

Co-authored-by: Alexander Richardson <mail@alexrichardson.me>
Signed-off-by: Tim Hutt <tdhutt@gmail.com>

model/riscv_pmp_control.sail

@@ -81,7 +81,7 @@ function pmpAddrRange(cfg: Pmpcfg_ent, pmpaddr: xlenbits, prev_pmpaddr: xlenbits
     TOR   => { Some ((prev_pmpaddr << 2, pmpaddr << 2)) },
     NA4   => {
                // NA4 is not selectable when the PMP grain G >= 1. See pmpWriteCfg().
-               assert(sys_pmp_grain() >= 1, "NA4 cannot be selected when PMP grain G >= 1.");
+               assert(sys_pmp_grain() < 1, "NA4 cannot be selected when PMP grain G >= 1.");
                let lo = pmpaddr << 2;
                Some((lo, lo + 4))
              },