wlcore: fix overlapping snprintf arguments in debugfs

gcc complains about undefined behavior in calling snprintf()
with the same buffer as input and output:

drivers/net/wireless/ti/wl18xx/debugfs.c: In function 'diversity_num_of_packets_per_ant_read':
drivers/net/wireless/ti/wl18xx/../wlcore/debugfs.h:86:3: error: 'snprintf' argument 4 overlaps destination object 'buf' [-Werror=restrict]
   86 |   snprintf(buf, sizeof(buf), "%s[%d] = %d\n",  \
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   87 |     buf, i, stats->sub.name[i]);   \
      |     ~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/wireless/ti/wl18xx/debugfs.c:24:2: note: in expansion of macro 'DEBUGFS_FWSTATS_FILE_ARRAY'
   24 |  DEBUGFS_FWSTATS_FILE_ARRAY(a, b, c, wl18xx_acx_statistics)
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/net/wireless/ti/wl18xx/debugfs.c:159:1: note: in expansion of macro 'WL18XX_DEBUGFS_FWSTATS_FILE_ARRAY'
  159 | WL18XX_DEBUGFS_FWSTATS_FILE_ARRAY(diversity, num_of_packets_per_ant,

There are probably other ways of handling the debugfs file, without
using on-stack buffers, but a simple workaround here is to remember the
current position in the buffer and just keep printing in there.

Fixes: bcca1bbd ("wlcore: add debugfs macro to help print fw statistics arrays")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210323125723.1961432-1-arnd@kernel.org