phy: phy-brcm-usb: Constify static structs

A number of structs were not modified and can therefore be made const
to allow the compiler to put them in read-only memory.

In order to do so, update a few functions that don't modify there input
to take pointers to const.

Before:
   text    data     bss     dec     hex filename
  15511    6448      64   22023    5607 drivers/phy/broadcom/phy-brcm-usb.o

After:
   text    data     bss     dec     hex filename
  16058    5936      64   22058    562a drivers/phy/broadcom/phy-brcm-usb.o

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Link: https://lore.kernel.org/r/20200516120441.7627-4-rikard.falkeborn@gmail.com
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>