net: ice: ptp: fix compilation warning if PTP_1588_CLOCK is disabled

Fix the following compilation warning if PTP_1588_CLOCK is not enabled

drivers/net/ethernet/intel/ice/ice_ptp.h:149:1:
   error: return type defaults to ‘int’ [-Werror=return-type]
   ice_ptp_request_ts(struct ice_ptp_tx *tx, struct sk_buff *skb)

Fixes: ea9b847c ("ice: enable transmit timestamps for E810 devices")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/ice/ice_ptp.h

@@ -145,7 +145,7 @@ static inline int ice_get_ptp_clock_index(struct ice_pf *pf)
 	return -1;
 }
 
-static inline
+static inline s8
 ice_ptp_request_ts(struct ice_ptp_tx *tx, struct sk_buff *skb)
 {
 	return -1;