ALSA: emu8000: Convert to generic PCM copy ops

This patch converts the SB Emu8000 driver code to use the new unified
PCM copy callback.  The conversion is a bit complicated because of
many open code in emu8000_pcm.c.  GET_VAL() and LOOP_WRITE() macros
were rewritten / simplified with copy_from_iter().  As
copy_from_iter() updates the internal offset value, we can drop the
corresponding part, too.

Link: https://lore.kernel.org/r/20230815190136.8987-7-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>