ASoC: audio-ext-clk: Change gpio default value to -1

During stability test, gpio_direction_output is
invoked with msm gpio 0 which is leading to PC NOC error
as audio is not expected to access this gpio.
Hence initialise gpio to -1 and check if gpio
is valid before using.

CRs-Fixed: 973438
Change-Id: I32d779974f4eb497c62035f7f46c10739ebcfe5f
Signed-off-by: Yeleswarapu Nagaradhesh <nagaradh@codeaurora.org>