phy: core: Update documentation syntax

Update the syntax used by the documentation of phy operation functions.
This is to unify the syntax with the newly added documentation.

Signed-off-by: Jules Maselbas <jmaselbas@kalray.eu>
Link: https://lore.kernel.org/r/20220407102108.24211-3-jmaselbas@kalray.eu
Signed-off-by: Vinod Koul <vkoul@kernel.org>