Move Rx/Tx routines to separate file.
Some functions:
* ena_restore_device,
* ena_destroy_device,
* ena_up,
* ena_down,
* ena_refill_rx_bufs
could be reused in upcoming netmap code in the driver. To make it
possible, they were moved to ena.h header.
Differential Revision: https://reviews.freebsd.org/D21933
Submitted by: Rafal Kozik <rk@semihalf.com>
Obtained from: Semihalf
Sponsored by: Amazon, Inc.
