wl3501_cs: min_t() cast truncates high bits

wrqu->encoding.length comes from the network administrator.  It's
size u16.  We want to limit "tocopy" to the smallest value of either
"len_keys", "wrqu->encoding.length" or 100.  But because .length
gets cast from u16 to u8 we might use a random, smaller value than
the was desired.  It's probably not very serious, but we may as well
fix it.

Btw, this is from code auditing and not from testing.  I don't know
if this affects anyone in real life.

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