tests/drivers/flash: fix TEST_AREA_MAX
tests/drivers/flash: fix TEST_AREA_MAX

There was mistake in parentheses application in
the expression which calculates the value.

Signed-off-by: Andrzej Puzdrowski <0222c24e1773dd88269ed0e559909ec9bea3604a@nordicsemi.no>

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