phy: phy-brcm-usb: specify init function format at struct level

This is slightly cleaner solution that assures noone assings a wrong
function to the pointer.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Link: https://lore.kernel.org/r/20201216143305.12179-2-zajec5@gmail.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>