ath9k: Fix modal EEPROM dump

we provide excess buffer size for 'simple_read_from_buffer'
for modal EEPROM dump. This results in trailing NULL bytes
at the end of EEPROM dump, fix this.

Cc: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Reported-by: Holger Schurig <holgerschurig@googlemail.com>
Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Tested-by: Holger Schurig <holgerschurig@gogglemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>