lib/igt_sysfs: Let igt_sysfs_read|write return -errno

In some cases debugfs or sysfs may return errors that we
want to check. Return -errno from helper functions to make
asserts easier.

v2: don't forget about EOF ret=0 (Chris)
    small re-write (Michal)

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
1 file changed