spi-tools: bump to version 0.8.1

This version doesn't use GIT_VERSION anymore, so the sed hook is no
longer necessary.

Fixes http://autobuild.buildroot.net/results/ed89fae4a722b5d2adff9826bb88e55e58af77f6



Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>