ASoC: sgtl5000: Use snd_soc_kcontrol_codec()

Since commit a7295267 ("ASoC: sgtl5000: add avc support") the
following kernel crash happens after running a 'reboot' command:

ALSA: Storing mixer settings...
[   20.031604] Unable to handle kernel paging request at virtual address fffffffe
[   20.039268] pgd = de2a0000
[   20.041999] [fffffffe] *pgd=8fffd861, *pte=00000000, *ppte=00000000
[   20.048387] Internal error: Oops: 80000007 [#1] SMP ARM

The function that takes a kcontrol parameter and returns the
codec that registered the control is snd_soc_kcontrol_codec(),
so use the correct function to fix the problem.

Fixes: a7295267 ("ASoC: sgtl5000: add avc support")
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Tested-by: Richard Leitner <richard.leitner@skidata.com>
Reviewed-by: Richard Leitner <richard.leitner@skidata.com>
Signed-off-by: Mark Brown <broonie@kernel.org>