staging:rtl8192u: Rename RxLastFragNum - Style

Rename the member variable RxLastFragNum to rx_last_frag_num. This
change clears the checkpatch issue with CamelCase naming.

The resulting edits are all coding style changes 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>