tests: spi_loopback: Increase size of async callback thread stack
tests: spi_loopback: Increase size of async callback thread stack

The asynchronous callback thread was overflowing its stack and causing a
bus/mpu fault when hardware stack checking is enabled. Increase the size
of the stack.

Signed-off-by: Maureen Helm <87a696db5547fa477bda9f9e0f00013c839c14d3@nxp.com>

kraj/zephyr,punitvara/zephyr,kraj/zephyr,punitvara/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,nashif/zephyr,ldts/zephyr,kraj/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,punitvara/zephyr,ldts/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,nashif/zephyr,ldts/zephyr,kraj/zephyr,nashif/zephyr,galak/zephyr,explora26/zephyr,finikorg/zephyr,finikorg/zephyr,explora26/zephyr,galak/zephyr,explora26/zephyr,explora26/zephyr,kraj/zephyr,Vudentz/zephyr,Vudentz/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr