staging:rtl8192u: Replace magic number 6 with ETH_ALEN - Style

The array size 6 represents the length of an Ethernet address so the
magic number has been replaced with the defined constant representing
that length.

This is a coding style change which should not impact runtime code
execution.

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