autotools: update style

- rename configure.in to configure.ac
- replace INCLUDES with AM_CPPFLAGS
- modernize AM_INIT_AUTOMAKE invocation

Signed-off-by: Patrick Welche <prlw1@cam.ac.uk>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
pcm: Insert linear-to-float conversion when rate or channel count is incorrect

This fixes a bug where snd_pcm_plug_insert_plugins fails when both
client and slave use format float, but the rate or channel count does
not match. I also removed some redundant code.

Signed-off-by: Maarten Baert <maarten-baert@hotmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>