greybus: db3-platform: get rid of redundant gpio tests

In apb_ctrl_get_devtree_data(), the value returned by
of_get_named_gpio() has a redundant test for a negative
return value.  GPIO numbers are non-negative, so this
test is redundant--testing gpio_is_valid() is sufficient.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>