staging:rtl8192u: Rename RF90_RADIO_PATH_E - Style

Rename the enumerated type RF90_RADIO_PATH_E to rf90_radio_path_e.
Whilst it is not flagged as an issue by checkpatch, types are meant
to be named in lowercase.

This change is purely a coding style change which should have no
impact on runtime code execution.

Signed-off-by: John Whitmore <johnfwhitmore@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>