staging: rtl8192e: Replace ?: with max

All get_key implementations return either -1 or small buffers, so
cast int->u16 is not a problem.

Signed-off-by: Mateusz Kulikowski <mateusz.kulikowski@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>