media: v4l2-ctrls-api: fix error handling for v4l2_g_ctrl()

As detected by Coverity, the error check logic at get_ctrl() is
broken: if ptr_to_user() fails to fill a control due to an error,
no errors are returned and v4l2_g_ctrl() returns success on a
failed operation, which may cause applications to fail.

Add an error check at get_ctrl() and ensure that it will
be returned to userspace without filling the control value if
get_ctrl() fails.

Fixes: 71c689dc ("media: v4l2-ctrls: split up into four source files")
Cc: stable@vger.kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>