putting an error pointer into strerror can be a bit confusing since
strerror takes a positive error code but PTR_ERR returns a negative
number, so we have to do strerror(-PTR_ERR(errp)). Some places got
this wrong already, so introduce a strerrorp function which directly
takes an error pointer.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>