An attempt to get a BUTTON control returns EACCES, not EINVAL.

BUTTON controls have V4L2_CTRL_FLAG_WRITE_ONLY set, and the documentation
for that flag correctly says that getting a write-only control returns
EACCES. It's the description of the BUTTON type that's wrong, so fix
that so they are consistent.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>