staging:rtl8192u: Replace magic number with defined constant - Style

Replace the magic number '8' with defined constant which is normally used
'MAX_DEV_ADDR_SIZE'.

This is 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>