pinctrl: realtek: rtd1625: remove unused group name spdif_sel

Remove the undefined "spdif_sel" from rtd1625_iso_spdif_in_coaxial_groups
to clean up the code.

Signed-off-by: Yu-Chun Lin <eleanor.lin@realtek.com>
Signed-off-by: Linus Walleij <linusw@kernel.org>