ath5k: don't change mac in eeprom_read_mac on error

Do not touch mac parameter passed to ath5k_eeprom_read_mac unless
we are sure we have correct address. I.e. when returning error, do
not change it.

While at it, use '= {}' compiler trick for memsetting mac_d.

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Cc: Luis R. Rodriguez <lrodriguez@atheros.com>
Acked-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>