firmware: Use kvmalloc for page tables

This is a minor optimization to use kvmalloc() variant for allocating
the page table for the SG-buffer.  They aren't so big in general, so
kmalloc() would fit often better.

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