ASoC: rt5645: Fix potential crash in jd function

If no one defined the rt5645->pdata.hp_det_gpio in coreboot/bios.
It will cause kernel to reboot because rt5645->pdata.hp_det_gpio
is 0. So it is worth to add a check in rt5645_jack_detect.

Signed-off-by: Bard Liao <bardliao@realtek.com>
Signed-off-by: Fang, Yang A <yang.a.fang@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>