phy: mediatek: add a new helper to update bitfield

Due to FIELD_PREP() macro can be used to prepare a bitfield value,
local ones can be remove; add the new helper to make bitfield update
easier.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Link: https://lore.kernel.org/r/20220920090038.15133-2-chunfeng.yun@mediatek.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>