pcm: dmix: assembly: refactor the PIC mode

pcm: dmix: assembly: refactor the PIC mode

Newer versions of GCC (> 5.0) accept that ebx is declared
in the clobber list even in PIC mode. Meanwhile, even
unlikely, the compiler may use ebx as base address of
one of the memory entry, making subsequent access to
them unreliable ('size', 'dst_step', 'src_step', 'sum_step').

Adding ebx in the clobber solves the problem.
By the way, the entry 'old_ebx' is no longer required.

Signed-off-by: Frédéric Recoules <frederic.recoules@orange.fr>
Signed-off-by: Takashi Iwai <tiwai@suse.de>