staging: rtl8188eu: remove unncessary asignment to cleanup long line

Instead of first asign 'wrqu.data.length = p - buff' use 'p - buff'
directly in min_t() in the subsequent asignment. Clears a line over
80 characters checkpatch warning.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>