ALSA: vx: Fix possible transfer overflow

The pseudo DMA transfer codes in VX222 and VX-pocket driver have a
slight bug where they check the buffer boundary wrongly, and may
overflow.  Also, the zero sample count might be handled badly for the
playback (although it shouldn't happen in theory).  This patch
addresses these issues.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=141541


Signed-off-by: Takashi Iwai <tiwai@suse.de>