USBNET: centralize computing of max rx/tx qlen

This patch centralizes computing of max rx/tx qlen, because:

- RX_QLEN()/TX_QLEN() is called in hot path
- computing depends on device's usb speed, now we have ls/fs, hs, ss,
so more checks need to be involved
- in fact, max rx/tx qlen should not only depend on device USB
speed, but also depend on ethernet link speed, so we need to
consider that in future.
- if SG support is done, max tx qlen may need change too

Generally, hard_mtu and rx_urb_size are changed in bind(), reset()
and link_reset() callback, and change mtu network operation, this
patches introduces the API of usbnet_update_max_qlen(), and calls
it in above path.

Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/usb/usbnet.h

@@ -34,6 +34,7 @@ struct usbnet {
 	struct mutex		phy_mutex;
 	unsigned char		suspend_count;
 	unsigned char		pkt_cnt, pkt_err;
+	unsigned short		rx_qlen, tx_qlen;
 
 	/* i/o info: pipes etc */
 	unsigned		in, out;
@@ -253,4 +254,6 @@ extern void usbnet_link_change(struct usbnet *, bool, bool);
 extern int usbnet_status_start(struct usbnet *dev, gfp_t mem_flags);
 extern void usbnet_status_stop(struct usbnet *dev);
 
+extern void usbnet_update_max_qlen(struct usbnet *dev);
+
 #endif /* __LINUX_USB_USBNET_H */