ixgbe: Re-enable the SFP laser during initialization

ixgbe_if_stop() disables the transmit laser on every 82599 SFP fiber
port, but the iflib initialization path did not re-enable it. Re-enable
the laser before deferred SFP module setup so interface
reinitialization cannot leave either single-speed or multispeed optics
dark.

The hardware wrapper is a no-op when laser control is unavailable. The
placement follows Intel ix-3.4.39; this version deliberately applies to
every SFP port affected by the stop path.

MFC after:	1 week

sys/dev/ixgbe/if_ix.c

@@ -1836,6 +1836,8 @@ ixgbe_config_link(if_ctx_t ctx)
 	sfp = ixgbe_is_sfp(hw);
 
 	if (sfp) {
+		/* ixgbe_if_stop() disables it on every 82599 SFP port. */
+		ixgbe_enable_tx_laser(hw);
 		sc->task_requests |= IXGBE_REQUEST_TASK_MOD;
 		iflib_admin_intr_deferred(ctx);
 	} else {