pinctrl: bcm2835: Silence uninit warning

The uninitialized variable would be caught by the BUG_ON()
logic below, but the kernel test robot cannot see that.
Silence the warning by initializing the variable.

Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/r/20211209134513.306212-1-linus.walleij@linaro.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>