mac80211: ath9k: handle error from copy_to_user in read_file_eeprom

mac80211: ath9k: handle error from copy_to_user in read_file_eeprom

Handle error from copy_to_user in read_file_eeprom.
Fix compilarion warning:
/__w/openwrt/openwrt/openwrt/build_dir/target-x86_64-openwrt-linux-musl_musl/linux-x86_64/backports-6.1.24/drivers/net/wireless/ath/ath9k/debug.c: In function 'read_file_eeprom':
/__w/openwrt/openwrt/openwrt/build_dir/target-x86_64-openwrt-linux-musl_musl/linux-x86_64/backports-6.1.24/drivers/net/wireless/ath/ath9k/debug.c:1451:17: error: ignoring return value of 'copy_to_user' declared with attribute 'warn_unused_result' [-Werror=unused-result]
 1451 |                 copy_to_user(user_buf, from, bytes);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>