Change function validate_tx_req_id() to inline in ENA driver

The function is in hot path of the driver (TX) and asking compiler for
making this function inline was changed for consistency and higher
readability.

Submitted by: Michal Krawczyk <mk@semihalf.com>
Reviewed by: rlibby, byenduri_gmail.com
Obtained from: Semihalf
Sponsored by: Amazon, Inc.
Differential Revision: https://reviews.freebsd.org/D12854