Staging: rt2860: Fix test in rt_ioctl_siwfrag()

The test always evaluated to true.

MIN_FRAG_THRESHOLD is defined 256,
MAX_FRAG_THRESHOLD is defined 2346

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>