rtl8712: replace min with min_t

I've changed the ugly casting here and used min_t() instead.  I
also changed the u16 to a u32 because ->network.Ssid.SsidLength
is 32 bits.  It doesn't make a difference, but truncating the
upper bits away is sloppy.

Signed-off-by: Przemo Firszt <przemo@firszt.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>