hm0360: _set_pll(): detect read_reg failure before writing PLL1CFG

The old code stored the raw return value of read_reg() in an 8-bit
variable and wrote it straight back to PLL1CFG.  A negative I²C/SCCB
error therefore became 0xFF (or similar), silently corrupting the
sensor’s clock tree while still returning “success”.

* Read PLL1CFG into an int (`ret`) and return immediately if `ret < 0`.
* Cast to uint8_t only after the error check, then proceed with the
  masked write.

This propagates read failures to the caller and guarantees we never
write garbage to the PLL register under fault conditions.

Author: RubenKelevra

sensors/hm0360.c

@@ -358,7 +358,12 @@ static int _set_pll(sensor_t *sensor, int bypass, int multiplier, int sys_div, i
         value = 0x00;
     }
 
-    pll_cfg = read_reg(sensor->slv_addr, PLL1CFG);
+    int ret = read_reg(sensor->slv_addr, PLL1CFG);
+    if (ret < 0) {
+        return ret;
+    }
+
+    pll_cfg = (uint8_t)ret;
     return write_reg(sensor->slv_addr, PLL1CFG, (pll_cfg & 0xFC) | value);
 }