SD: Implement sdhc_card_busy in SDHC SPI-driver

Currently, the sdhc_card_busy function is unimplemented in the
SDHC SPI-driver. This causes some functions which rely on f_sync(),
such as fs_close(), to fail as it will timeout the busy-check. This PR
implements sdhc_card_busy by checking if the MISO-line is kept high to
check if the SD-card is idle (not busy).

Solves https://github.com/zephyrproject-rtos/zephyr/issues/49982



Signed-off-by: Ivan Herrera Olivares <ivan.herreraolivares@uantwerpen.be>