ath9k_hw: index out of bounds

Check whether index is within bounds before testing the element

Both spurChans arrays in modalHeader5G and modalHeader2G have 5 elements,
AR_EEPROM_MODAL_SPURS is defined 5. So unless a break occurs, in the
last iteration (i=5) we tried to access spurChansPtr[5] before testing
whether i was within bounds. Fix this.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>