staging: rtl8192e: avoid CamelCase <RATRIndex>

Linux kernel coding-style suggests to not use mixed-case names. Fix
checkpatch issue by changing the variable name from camel case to snake
case.

Signed-off-by: John Grace <johnmgrace1@gmail.com>
Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com>
Link: https://lore.kernel.org/r/ZEcjdUR/bnln7Z1J@iris


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>