[hyperv] Increase TX ring size

Empirical observation suggests that 32 is a sensible size to minimise
the number of deferred packet transmissions without overflowing the
VMBus transmit ring buffer.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/drivers/net/netvsc.h

  * must be sufficiently small to guarantee that we never run out of
  * space in the VMBus outbound ring buffer.
  */
-#define NETVSC_TX_NUM_DESC 8
+#define NETVSC_TX_NUM_DESC 32
 
 /** RX data buffer page set ID
  *