maemo: Fix a few crashing bugs

maemo: Fix a few crashing bugs

maemo plugin has two crashes I was able to see in a valgrind log from
another user:

* maximum write size was calculated in words (16bit), but checked against
  byte-size length. This causes memcpy later to overflow the buffer
  (normally by up to 12KB).
* remove a double free (by marking free'd data with NULL)

* mmap returns MMAP_FAILED on error, not NULL

I suspect that this plugin/driver might have other issues aswell, since I
am unable to find any logic for checking DSP buffer status, and no
implementation for odelay reporting.

Signed-off-by: Stian Skjelstad <stian.skjelstad@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>