drivers: flashdisk: use instance specific buffers
drivers: flashdisk: use instance specific buffers

Allocate cache buffer for every flashdisk instance instead of using one
globally shared buffer.

Signed-off-by: Tomasz Moń <f44a0a0857de22882044c93edbe6b590cb840e13@nordicsemi.no>

zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr