pcm: file: Enable file writing for capture path

pcm: file: Enable file writing for capture path

This commit reverts parts of commit 4081be0b87ab9fa53a8906e66bc240f18a7a9a54,
because it is realy useful to use the file plugin in a capture path for
debugging. Also it fixes the truncate issue mentioned in above commit.

Additionally following MMAP access issue is considered:
$ arecord -D teeraw -M -d5 arecord.wav
Recording WAVE 'arecord.wav' : Unsigned 8 bit, Rate 8000 Hz, Mono
ALSA lib pcm/pcm_file.c:358:(snd_pcm_file_write_bytes)
write failed: Bad file descriptor
ALSA lib pcm/pcm_file.c:358:(snd_pcm_file_write_bytes)
write failed: Bad file descriptor
arecord: pcm/pcm_file.c:397: snd_pcm_file_add_frames:
Assertion `file->wbuf_used_bytes < file->wbuf_size_bytes' failed.
Aborted by signal Aborted...

Signed-off-by: Timo Wischer <twischer@de.adit-jv.com>
Signed-off-by: Mounesh Sutar <sutar.mounesh@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>