chore(bhi160): Move FIFO statics into function body

These two static variables aren't (and shouldn't!) be used anywhere
outside of bhi160_fetch_fifo() so let's move them into the function.

Signed-off-by: Rahix <rahix@rahix.de>

epicardium/modules/bhi.c

@@ -44,10 +44,6 @@ static float bhi160_sic_array[3 * 3] = { 1.0, 0.0, 0.0,
                                          0.0, 0.0, 1.0 };
 /* clang-format on */
 
-/* BHI160 Fifo */
-static uint8_t bhi160_fifo[BHI160_FIFO_SIZE];
-static size_t start_index = 0;
-
 /* BHI160 Task ID */
 static TaskHandle_t bhi160_task_id = NULL;
 
@@ -348,6 +344,13 @@ static void bhi160_fetch_fifo(void)
 	 * You'll probably be best of leaving it as it is ...
 	 */
 
+	/*
+	 * FIFO buffer.  Access to this static variable is safe because this
+	 * function is guarded by the bhi160_mutex.
+	 */
+	static uint8_t bhi160_fifo[BHI160_FIFO_SIZE];
+	static size_t start_index = 0;
+
 	int result = 0;
 	/* Number of bytes left in BHI160's FIFO buffer */
 	uint16_t bytes_left_in_fifo = 1;