net: rtl8139: Read HW address from EEPROM only on probe

Do not re-read the HW address from the EEPROM on every start of
transfer, otherwise the user will not be able to adjust ethaddr
as needed. Read the address only once, when the card is detected.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>