mmc: renesas_sdhi: really fix WP logic regressions

This reverts commit e060d376 ("mmc: renesas_sdhi: fix WP detection")
and adds some code to really fix the regressions.

It was missed so far that Renesas R-Car instantiations of SDHI chose to
disable internal WP and used the existence of "wp-gpios" to en/disable
WP at all.

With the first refactoring by Yamada-san with commit 2ad1db05 ("mmc:
renesas_sdhi: use MMC_CAP2_NO_WRITE_PROTECT instead of TMIO own flag"),
WP was always disabled even when GPIOs were present. With Wolfram's
first fix which gets now reverted, GPIOs were honored. But when not
available, the fallback was to internal WP and not to disabled WP. This
caused wrong WP status on uSD card slots.

Restore the old behaviour now. By default, WP is disabled. When a GPIO
is found, the GPIO re-enables WP. We will think about possible better
ways to handle this in the future.

Tested on a previously regressing Renesas Lager board (H2) and a still
working Renesas Salvator-X board (M3-W).

Reported-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Cc: stable@vger.kernel.org # v4.17+
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>