alsa: Use 8 bit pointer type for byte-based pointer arithmetic

alsa: Use 8 bit pointer type for byte-based pointer arithmetic

Usually these loops only run once, so there's no problem here. But sometimes
they run twice, and by adding the number of bytes to a 16 bit pointer type we
would advance twice as much as we should.

Also use snd_pcm_frames_to_bytes() in alsasrc to calculate
the number of bytes to skip, same as we do in alsasink.

Thanks to Lucio A. Hernandez <lucio.a.hernandez@gmail.com> for reporting.