net: phy: broadcom: don't export RDB/legacy access methods

Don't export __bcm_phy_enable_rdb_access() and
__bcm_phy_enable_legacy_access() functions. They aren't used outside this
module and it was forgotten to provide a prototype for these functions.
Just make them static for now.

Fixes: 11ecf8c5 ("net: phy: broadcom: add cable test support")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>