net: dsa: mv88e6xxx: Make SMI c22/c45 read/write functions static

The SMI clause 22 & 45 read/write operations are local to the global2.c file,
so make them static. This eliminates the following warning:

drivers/net/dsa/mv88e6xxx/global2.c:571:5: warning: no previous prototype for 'mv88e6xxx_g2_smi_phy_read_c45' [-Wmissing-prototypes]
 int mv88e6xxx_g2_smi_phy_read_c45(struct mv88e6xxx_chip *chip, int addr,
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/dsa/mv88e6xxx/global2.c:602:5: warning: no previous prototype for 'mv88e6xxx_g2_smi_phy_read_c22' [-Wmissing-prototypes]
 int mv88e6xxx_g2_smi_phy_read_c22(struct mv88e6xxx_chip *chip, int addr,
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/dsa/mv88e6xxx/global2.c:635:5: warning: no previous prototype for 'mv88e6xxx_g2_smi_phy_write_c45' [-Wmissing-prototypes]
 int mv88e6xxx_g2_smi_phy_write_c45(struct mv88e6xxx_chip *chip, int addr,
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/dsa/mv88e6xxx/global2.c:664:5: warning: no previous prototype for 'mv88e6xxx_g2_smi_phy_write_c22' [-Wmissing-prototypes]
 int mv88e6xxx_g2_smi_phy_write_c22(struct mv88e6xxx_chip *chip, int addr,
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>