RP2040: Adjust sync 5MB/s mode timings

In synchronous 5MB/s mode, the REQ negation period used to be >= 102 ns.
After the RP2040 clocking updates in October 2024, the negation period is >= 90 ns.
That is just barely within SCSI-2 specification.

This commit increases the negation period slightly to 100 ns.

This affects only the slowest synchronous mode, as fast SCSI supports negation
period down to 30 ns.

lib/ZuluSCSI_platform_RP2MCU/scsi_accel_target.cpp

@@ -1272,11 +1272,12 @@ bool scsi_accel_rp2040_setSyncMode(int syncOffset, int syncPeriod)
             else
             {
                 // Slow SCSI timing: 90 ns assertion period, 55 ns skew delay
+                // Delay2 must be at least 2 to keep negation period well above the 90 ns minimum
                 totalPeriod += g_zuluscsi_timings->scsi_5.total_period_adjust;
                 delay0 = g_zuluscsi_timings->scsi_5.delay0;
                 delay1 = g_zuluscsi_timings->scsi_5.delay1;
                 delay2 = totalPeriod - delay0 - delay1 - 3;
-                if (delay2 < 0) delay2 = 0;
+                if (delay2 < 2) delay2 = 2;
                 if (delay2 > 15) delay2 = 15;
                 rdelay1 = g_zuluscsi_timings->scsi_5.rdelay1;
                 rtotalPeriod += g_zuluscsi_timings->scsi_5.rtotal_period_adjust;
@@ -1303,4 +1304,4 @@ bool scsi_accel_rp2040_setSyncMode(int syncOffset, int syncPeriod)
     }
 
     return true;
-}
+}