ath5k: snprintf() returns largish values

snprintf() returns the number of characters that would have been written
(not counting the NUL character).  So we can't use it as the limiter to
simple_read_from_buffer() without capping it first at sizeof(buf).

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>