ALSA: snd_pcm_oss_period_size: Use round{up,down}_pow_of_two()

Instead of opencoding them use the standard roundup_pow_of_two() and
rounddown_pow_of_two() helper functions. This gets rids one of the few users
of the custom ld2() function and also makes it a bit more obvious what the
code does.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>