staging: rtl8192u: make r8192_wx_handlers_def structure const

Make this const as it is only stored in a const field of a pci_dev
structure. Make the declaration in the header const too.

Structure found using Coccinelle and changes done by hand.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>