drivers: disk: sdmmc_stm32: power up on request

Move power configuration from the device init function into the
`DISK_IOCTL_CTRL_INIT` and `DISK_IOCTL_CTRL_DEINIT` handlers. If
`pwr-gpios` is defined, this ensures that the card is not powered up
until requested, and enables power down the card when not in use.

This behaviour matches the implementation for `sdhc_spi`.

Signed-off-by: Jordan Yates <jordan@embeint.com>