net: ena: PHC: Add missing barrier

Add dma_rmb() barrier after req_id completion check in
ena_com_phc_get_timestamp(). On weakly-ordered architectures,
payload fields may be read before req_id is observed as updated.

Fixes: e0ea34158ee8 ("net: ena: Add PHC support in the ENA driver")
Closes: https://sashiko.dev/#/patchset/20260430032507.11586-1-akiyano%40amazon.com
Signed-off-by: Arthur Kiyanovski <akiyano@amazon.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>