pinctrl: th1520: Factor out casts

Limit the casts to get the mux data and flags from the driver data
pointer with each pin to two inline functions as requested by Andy
during review.

Signed-off-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Reviewed-by: Drew Fustini <dfustini@tenstorrent.com>
Tested-by: Drew Fustini <dfustini@tenstorrent.com>
Link: https://lore.kernel.org/20241011144826.381104-4-emil.renner.berthing@canonical.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>