ath9k: Add a #define for the EEPROM "eepmisc" endianness bit

This replaces a magic number with a named #define. Additionally it
removes two "eeprom format" specific #defines for the "big endianness"
bit which are the same on all eeprom formats.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>