dshot: reduce static buffer size

And handle failures of up_dshot_init().

On Omnibus: reduces memory usage if dshot is enabled by ~1.0KB.
The buffer is roughly 1KB in size.

boards/nxp/fmuk66-v3/src/board_config.h

@@ -520,6 +520,8 @@ __BEGIN_DECLS
 
 #define BOARD_HAS_NOISY_FXOS8700_MAG 1 // Disable internal MAG
 
+#define BOARD_NUM_IO_TIMERS 3
+
 /************************************************************************************
  * Public data
  ************************************************************************************/