wl18xx: show rx_frames_per_rates as an array as it really is

In struct wl18xx_acx_rx_rate_stat, rx_frames_per_rates field is an
array, not a number.  This means WL18XX_DEBUGFS_FWSTATS_FILE can't be
used to display this field in debugfs (it would display a pointer, not
the actual data).  Use WL18XX_DEBUGFS_FWSTATS_FILE_ARRAY instead.

This bug has been found by adding a __printf attribute to
wl1271_format_buffer.  gcc complained about "format '%u' expects
argument of type 'unsigned int', but argument 5 has type 'u32 *'".

Fixes: c5d94169

 ("wl18xx: use new fw stats structures")
Signed-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>