rtl818x_pci: add comment pointing to the rtl8187se reference code

Rtl8187se support has been added to the rtl818x_pci driver by extracting a lot
of information from a rtl8187se Linux staging driver included in the kernel at
the time rtl8187se support was added.
The rtl818x_pci main file has a comment that advertises this.

Recently this staging driver has been removed from the kernel, but I still feel
it can be useful as "reference" code (in case of bugs, or to implement
improvements in rtl818x_pci driver).

This one-line patch adds a comment in rtl818x_pci driver to point people
searching for that "reference code" to the last kernel version still containing
it (3.14).

Signed-off-by: Andrea Merello <andrea.merello@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>