ipwireless: Preallocate received packet buffers with MRU size

Packets are assembled from link size (~300 bytes) up to PPP MRU
(1500 by default). Try to preallocate full size rather than
repeatedly advance buffer size by 256 bytes.
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>