rt2x00: Export rt2x00soc_probe from rt2x00soc

Export rt2x00soc_probe from rt2x00soc as it is used in rt2800pci.

Otherwise loading rt2800pci gives "rt2800pci: Unknown symbol
rt2x00soc_probe".

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>