Staging: rtl8712: Replace __constant_cpu_to_le16

This fixes the following checkpatch.pl warning:
WARNING: __constant_cpu_to_le16 should be cpu_to_le16

Signed-off-by: Dilek Uzulmez <dilekuzulmez@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>