samples: flash_shell: Replace incorrect assignment to boolean expression
samples: flash_shell: Replace incorrect assignment to boolean expression

`shell_cmd_precheck()` function has the second argument as boolean
and currently we pass assignment expression instead of boolean
expression. Therefore, fix the expression by passing `argc == 2`
as the boolean argument.

Fixes #11099
Coverity-CID: 189507

Signed-off-by: Himanshu Jha <db39123178a3996d9dafcdcc9af59c3e841de70a@gmail.com>

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