ipw2x00: clean up a condition

The original condition was "(PAGE_SIZE - len)" when "(len < PAGE_SIZE)"
is intended.

This condition is not really sufficient, but also not really needed...
If "len > PAGE_SIZE" then it we will print a warning message in dmesg
but there are no other effects.  Maybe we should just remove the
condition?

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