hw/mcu/native: Fix hal_timer_get_resolution

| Error: repos/apache-mynewt-core/hw/mcu/native/src/hal_timer.c:
 In function ‘hal_timer_get_resolution’:
| repos/apache-mynewt-core/hw/mcu/native/src/hal_timer.c:154:24:
    error: array subscript -1 is below array bounds of
    ‘struct native_timer[1]’ [-Werror=array-bounds]
|   154 |     nt = &native_timers[num];
|       |           ~~~~~~~~~~~~~^~~~~
| repos/apache-mynewt-core/hw/mcu/native/src/hal_timer.c:41:3:
     note: while referencing ‘native_timers’
|    41 | } native_timers[1];
|       |   ^~~~~~~~~~~~~
| cc1: all warnings being treated as errors
diff --git a/hw/mcu/native/src/hal_timer.c b/hw/mcu/native/src/hal_timer.c
index 77d3d0e..59d091c 100644
--- a/hw/mcu/native/src/hal_timer.c
+++ b/hw/mcu/native/src/hal_timer.c
@@ -148,7 +148,7 @@
 {
     struct native_timer *nt;
 
-    if (num >= 0) {
+    if (num != 0) {
         return 0;
     }
     nt = &native_timers[num];