net: phy: export phy_{read,write}_mmd_indirect

Some PHY drivers might need to access Clause 45 registers in Clause 22
compatibility mode to e.g: properly advertise EEE support when disabled
by default.

Export these two helper functions: phy_read_mmd_indirect() and
phy_write_mmd_indirect() for drivers to use them.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>