ASoC: rt5645: Remove unused rt5645 variable

"ASoC: rt5645: Simplify rt5645_enable_push_button_irq" removes
the test that accessed rt5645->pdata.jd_mode (that test is now
done in rt5645_jack_detect only), so we do not need that
variable anymore.

Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
Signed-off-by: Mark Brown <broonie@kernel.org>