phy: phy-bcm-ns2-usbdrd: 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
   7831    3144     128   11103    2b5f drivers/phy/broadcom/phy-bcm-ns2-usbdrd.o

After:
   text    data     bss     dec     hex filename
   7959    3016     128   11103    2b5f drivers/phy/broadcom/phy-bcm-ns2-usbdrd.o

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Link: https://lore.kernel.org/r/20200516120441.7627-2-rikard.falkeborn@gmail.com
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>