broadcom: bcm54xx_shadow_read() errors ignored in bcm54xx_adjust_rxrefclk()

If not signed read errors are ignored.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Acked-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>