mmc: core: Don't return an error for CD/WP GPIOs when GPIOLIB is unset

When CONFIG_GPIOLIB is unset, its stubs will return -ENOSYS. That means
when the mmc core parses DT for CD/WP GPIOs via mmc_of_parse(), -ENOSYS
becomes propagated to the caller. Typically this means that the mmc host
driver fails to probe.

As the CD/WP GPIOs are already treated as optional, let's extend that to
cover the case when CONFIG_GPIOLIB is unset.

Reported-by: Michal Simek <michal.simek@xilinx.com>
Fixes: 16b23787

 ("mmc: sdhci-of-arasan: Call OF parsing for MMC")
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Venu Byravarasu <vbyravarasu@nvidia.com>