pinctrl: thunderbay: rework loops looking for groups names

pinctrl: thunderbay: rework loops looking for groups names

Make the outer loop iterate over functions as that's the real subject.
This simplifies code (and reduces amount of lines of code) as allocating
memory for names doesn't require extra checks anymore.

While at it use local "group_names" variable. It fixes:
drivers/pinctrl/pinctrl-thunderbay.c: In function 'thunderbay_add_functions':
drivers/pinctrl/pinctrl-thunderbay.c:815:8: warning: assignment discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
  815 |    grp = func->group_names;
      |        ^

Ref: 491dec224fe2 ("pinctrl: keembay: rework loops looking for groups names")
Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Link: https://lore.kernel.org/r/20220111172919.6567-2-zajec5@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>