spi: mpc8xxx_spi.c: fix cs activate/deactivate

spi: mpc8xxx_spi.c: fix cs activate/deactivate

Somewhere between v2020.04 and v2020.07 the mpc8xxx_spi driver broke,
I'm guessing due to this hunk

@@ -559,6 +560,8 @@ int dm_gpio_set_dir_flags(struct gpio_desc *desc, ulong flags)
        if (ret)
                return ret;

+       /* combine the requested flags (for IN/OUT) and the descriptor flags */
+       flags |= desc->flags;
        ret = _dm_gpio_set_dir_flags(desc, flags);

from commit 695e5fd5469a ("gpio: update dir_flags management"). But
the blame is mostly on the driver itself which seems rather confused:
The chip select gpios are requested with GPIOD_ACTIVE_LOW, but then in
each activate/deactivate, dm_gpio_set_dir_flags() is called with
merely GPIOD_IS_OUT, and then the driver call set_value(0) for
activate.

That used to work, but with the above hunk, the ACTIVE_LOW setting
from the request becomes persistent, so the gpio driver ends up being
asked to set the value to 1 in mpc8xxx_spi_cs_activate().

So drop the dm_gpio_set_dir_flags() calls in the activate/deactivate
functions, and use a value of 1 to mean "logically enabled".

Ideally, I think we should also drop the GPIOD_ACTIVE_LOW from the
request and make it up to the list of gpio cs in DT to indicate
whether that CS is enabled when driven low (as is of course usually
the case), but that requires changing
arch/powerpc/dts/gdsys/gazerbeam-base.dtsi among others, and I don't
have that hardware to test on. I have, however, tested our
own (mpc8309-based) hardware with this change, and I have also tested
that removing the GPIOD_ACTIVE_LOW from the request and updating our
DT as

-                       gpios = <&spisel 0 0>;
+                       gpios = <&spisel 0 GPIO_ACTIVE_LOW>;

still works.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>