cortexm/detach: Add a dummy transaction after cleaning DHCSR.

This replaces the seemingly superflous swdptap_seq_out() at
the end of adiv5_swdp_low_access() needed to continue after detach.

src/target/cortexm.c

@@ -413,6 +413,8 @@ void cortexm_detach(target *t)
 
 	/* Disable debug */
 	target_mem_write32(t, CORTEXM_DHCSR, CORTEXM_DHCSR_DBGKEY);
+	/* Add some clock cycles to get the CPU running again.*/
+	target_mem_read32(t, 0);
 }
 
 enum { DB_DHCSR, DB_DCRSR, DB_DCRDR, DB_DEMCR };