net: phy: marvell: make some functions static

functions m88e1510_get_temp_critical, m88e1510_set_temp_critical and
m88e1510_get_temp_alarm can be made static as they not need to be
in global scope.

Cleans up sparse warnings:
 "symbol 'm88e1510_get_temp_alarm' was not declared. Should it be static?"
 "symbol 'm88e1510_get_temp_critical' was not declared. Should it be
  static?"

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>