net: sierra_net: rx_urb_size is constant

The rx_urb_size is set to the same value for every device
supported by this driver.  No need to keep a per-device
data structure to do that. Replacing with a macro constant.

This was the last device specific info, and removing it
allows us to delete the sierra_net_info_data struct.

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>