Split Rx/Tx from initialization code in ENA driver

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.