ath9k: check pdata variable before dereferencing it

Due to my recent commit (ath9k: allow to load EEPROM
content via firmware API) smatch complains about that
the 'pdata' variable in 'ath9k_hw_init' can be NULL
and it is dereferenced before checking that. That is
absolutely correct.

Check the 'pdata' variable before using it to avoid
a NULL pointer dereference.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>