ath10k: cache calibration data when the core is stopped

Commit 0b8e3c4ca29f ("ath10k: move cal data len to hw_params") broke retrieving
the calibration data from cal_data debugfs file. The length of file was always
zero. The reason is:

    static ssize_t ath10k_debug_cal_data_read(struct file *file,
                                          char __user *user_buf,
                                          size_t count, loff_t *ppos)
    {
        struct ath10k *ar = file->private_data;
        void *buf = file->private_data;

This is obviously bogus, private_data cannot contain both struct ath10k and the
buffer. Fix it by caching calibration data to ar->debug.cal_data. This also
allows it to be accessed when the device is not active (interface is down).

The cal_data buffer is fixed size because during the first firmware probe we
don't yet know what will be the lenght of the calibration data. It was simplest
just to use a fixed length. There's a WARN_ON() in
ath10k_debug_cal_data_fetch() if the buffer is too small.

Tested with qca988x and firmware 10.2.4.70.56.

Reported-by: Nikolay Martynov <mar.kolya@gmail.com>
Fixes: 0b8e3c4ca29f ("ath10k: move cal data len to hw_params")
Cc: stable@vger.kernel.org # 4.7+
Signed-off-by: Marty Faltesek <mfaltesek@google.com>
[kvalo@qca.qualcomm.com: improve commit log and minor other changes]
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>