ath5k: Use scnprintf() for avoiding potential buffer overflow
Since snprintf() returns the would-be-output size instead of the
actual output size, the succeeding calls may go beyond the given
buffer limit. Fix it by replacing with scnprintf().
Cc: Jiri Slaby <jirislaby@gmail.com> Cc: Nick Kossifidis <mickflemm@gmail.com> Cc: Luis Chamberlain <mcgrof@kernel.org> Signed-off-by: Takashi Iwai <tiwai@suse.de> Signed-off-by: Kalle Valo <kvalo@codeaurora.org>