p54: potential signedness issue in p54_parse_rssical()

"entries" is unsigned here, so it is never less than zero.  In theory,
len could be less than offset so I have added a check for that.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>