rt2x00: Add dynamic detection of eFuse EEPROM in rt2800pci.

Instead of assuming that all rt3090 devices will have an eFuse EEPROM,
do as the legacy Ralink driver, and detect at run-time whether an
eFuse EEPROM is present.

Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>