rtl8180: move eeprom read stuff in a separate function

Eeprom read operations are mixed in the probe function.
Make the code more readable and clean by extracting this code and
moving it in a dedicated function.

The variable eeprom_cck_table_adr, now useless, is here because
it will be needed for rtl8187se support, that I hope to add soon.

Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>