staging: r8188eu: cleanup by using "len" consistently

"*(p + 1)" and "len" are the same thing.  For reviewers who don't know
that, then this code is worrying because we cap "len", but pass
"*(p + 1)" to memcpy().

I have changed the code to use "len" throughout.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>