pinctrl: stmfx: Fix hazardous u8[] to unsigned long cast

Casting a small array of u8 to an unsigned long is *never* OK:

- it does funny thing when the array size is less than that of a long,
  as it accesses random places in the stack
- it makes everything even more fun with a BE kernel

Fix this by building the unsigned long used as a bitmap byte by byte,
in a way that works across endianess and has no undefined behaviours.

An extra BUILD_BUG_ON() catches the unlikely case where the array
would be larger than a single unsigned long.

Fixes: 1490d9f8 ("pinctrl: Add STMFX GPIO expander Pinctrl/GPIO driver")
Signed-off-by: Marc Zyngier <maz@kernel.org>
Cc: stable@vger.kernel.org
Cc: Amelie Delaunay <amelie.delaunay@foss.st.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Alexandre Torgue <alexandre.torgue@foss.st.com>
Link: https://lore.kernel.org/r/20210725180830.250218-1-maz@kernel.org


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