p54: fix a fw crash caused by statistic feedback

This patch fixes a bug which crawled into the tree with the split-up
changes.

The memory-manager wasn't aware of the statistic feedback
extra_len space requirements and happily placed following frames
into the allegedly free spots.

Thanks fly out to Larry Finger for taking the time to
test all (permutations of) patches and theories all day long.

Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Christian Lamparter <chunkeey@web.de>
Tested-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>