ena: Use correct READ_ONCE on u8 field

[osamaabb_amazon.com]

Summary

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 was stored in cdesc_phase, but this change makes it
even more accurate.

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

[cperciva]

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.

Saying "the correct value is being read" sounds wrong if we're reading 16 bits from an 8-bit field. I think you mean "the correct value was stored in cdesc_phase"?

[osamaabb_amazon.com]

In D46336#1064445, @cperciva wrote:

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.

Saying "the correct value is being read" sounds wrong if we're reading 16 bits from an 8-bit field. I think you mean "the correct value was stored in cdesc_phase"?

Ack