rt2x00: adjust EEPROM_SIZE for rt2500usb

rt2500usb_validate_eeprom() read data up to 0x6e (EEPROM_CALIBRATE_OFFSET)
but only 0x6a bytes has been allocated and read from the eeprom.

This lead to out-of-bound accesses and invalid values for
EEPROM_BBPTUNE_R17 and EEPROM_CALIBRATE_OFFSET.

Change the EEPROM_SIZE to 0x6e in order to retrieve all the fields.

Tested with a rt2570 device.

Signed-off-by: Adrien Schildknecht <adrien+dev@schischi.me>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>