[ALSA] es1938 - improve capture hw pointer reads

With the Solo1 (es1938) I got a lot of xrun's during capture on my machine.
Tracing that down it seems to be comming from reading ocassionaly bad hw
pointers from the chip.  This patch uses more checking to avoid that false
pointer reads.
Failed reads are giving back the last good value read instead of spinning in
a tight loop, which seems more appropriate to me in an interrupt. I think I
saw this trick used in another driver

Signed-off-by: Hermann Lauer <Hermann.Lauer@iwr.uni-heidelberg.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>