Previous commit has enabled buffers in PSRAM for ESP32-P4. But this
also caused a regression for ESP32-S3, where PSRAM is not DMA capable.
This commit re-introduces the check for esp_ptr_external_ram in case
SOC_SDMMC_PSRAM_DMA_CAPABLE is not set.
This simplifies the code a bit,
- removing SOC_SDMMC_IO_POWER_EXTERNAL ifdefs from error paths
- moving the LDO code next to where sdmmc_test_board_card_power_set
is called for the same purpose