ath79/mikrotik: add function for loading ath9k caldata

The commands to read ath9k caldata on mikrotik subtarget are
mostly repetitive, so let's put them into a function to make
writing and reading them easier.

This function will only be required when patching the MAC address.
For cases where it is put correctly into the calibration data by
the vendor, caldata_sysfsload_from_file can be used directly as
done for ath10k at the moment.

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>