usb: f_ncm: only first packet of aggregate needs to start timer

The reasoning for this change is that if we already had
a packet pending, then we also already had a pending timer,
and as such there is no need to reschedule it.

This also prevents packets getting delayed 60 ms worst case
under a tiny packet every 290us transmit load, by keeping the
timeout always relative to the first queued up packet.
(300us delay * 16KB max aggregation / 80 byte packet =~ 60 ms)

As such the first packet is now at most delayed by 300us.

Under low transmit load, this will simply result in us sending
a shorter aggregate, as originally intended.

This patch has the benefit of greatly reducing (by ~10 factor
with 1500 byte frames aggregated into 16 kiB) the number of
(potentially pretty costly) updates to the hrtimer.

Cc: Brooke Basile <[e-mail]>
Cc: Bryan O'Donoghue <[e-mail]>
Cc: Felipe Balbi <[e-mail]>
Cc: Lorenzo Colitti <[e-mail]>
Signed-off-by: Maciej Żenczykowski <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Cc: stable <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/gadget/function/f_ncm.c

···
 			ncm->ndp_dgram_count = 1;
 
 			/* Note: we skip opts->next_ndp_index */
-		}
 
-		/* Delay the timer. */
-		hrtimer_start(&ncm->task_timer, TX_TIMEOUT_NSECS,
-			      HRTIMER_MODE_REL_SOFT);
+			/* Start the timer. */
+			hrtimer_start(&ncm->task_timer, TX_TIMEOUT_NSECS,
+				      HRTIMER_MODE_REL_SOFT);
+		}
 
 		/* Add the datagram position entries */
 		ntb_ndp = skb_put_zero(ncm->skb_tx_ndp, dgram_idx_len);