ena: Use correct READ_ONCE on u8 field

The flags field in ena_eth_io_tx_cdesc is 8-bits long.
The current macro used is READ_ONCE16.
Switching to READ_ONCE8 to avoid reading extra data.
Given that there's an implicit cast to u8 in the assignment,
the correct value is being read, but this change makes it
even more accurate.

Approved by: cperciva (mentor)
MFC after: 2 weeks
Sponsored by: Amazon, Inc.