The code for .get_multiple() has bugs:

1. The simple .get_multiple() just reads a register, masks it
and sets the return value. This is not correct: we only want to
assign values (whether 0 or 1) to the bits that are set in the
mask. Fix this by using &= ~mask to clear all bits in the mask
and then |= val & mask to set the corresponding bits from the
read.

2. The bgpio_get_multiple_be() call has a similar problem: it
uses the |= operator to set the bits, so only the bits in the
mask are affected, but it misses to clear all returned bits
from the mask initially, so some bits will be returned
erroneously set to 1.

3. The bgpio_get_set_multiple() again fails to clear the bits
from the mask.

4. find_next_bit() wasn't handled correctly, use a totally
different approach for one function and change the other
function to follow the design pattern of assigning the first
bit to -1, then use bit + 1 in the for loop and < num_iterations
as break condition.

Fixes: 80057cb4 ("gpio-mmio: Use the new .get_multiple() callback")
Cc: Bartosz Golaszewski <brgl@bgdev.pl>
Reported-by: Clemens Gruber <clemens.gruber@pqgruber.com>
Tested-by: Clemens Gruber <clemens.gruber@pqgruber.com>
Reported-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>