staging:rtl8192u: Rename RxLastSeqNum - Style

Rename the member variable RxLastSeqNum to rx_last_seq_num. This change
clears the checkpatch issue with CamelCase naming.

The resulting changes are purely coding style in nature so 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>