firmware: Free temporary page table after vmapping

Once after performing vmap() to map the S/G pages, our own page table
becomes superfluous since the pages can be released via vfree()
automatically.  Let's change the buffer release code and discard the
page table array for saving some memory.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>