linux: don't check hashes for user-supplied patches

We have virtually no way to know the hashes for user-supplied patches,
so we should just ignore them.

Reported-by: Simon van der Veldt <simon.vanderveldt@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Peter Korsgaard <peter@korsgaard.com>
Tested-by: Simon van der Veldt <simon.vanderveldt@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>