phy: sr-usb: Constify phy_ops

phy_ops are never modified and can therefore be made const to allow the
compiler to put it in read-only memory.

Before:
   text    data     bss     dec     hex filename
   4310    1244       0    5554    15b2 drivers/phy/broadcom/phy-bcm-sr-usb.o

After:
   text    data     bss     dec     hex filename
   4438    1116       0    5554    15b2 drivers/phy/broadcom/phy-bcm-sr-usb.o

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Link: https://lore.kernel.org/r/20200516120441.7627-3-rikard.falkeborn@gmail.com
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>