wifi: nrf70: Fix system work queue stack size override

Defaults cannot be overidden (even with configdefault extension), they
only work if the original symbol is defined after the override e.g.,
"drivers and then subsys/net" but for kernel symbols,the default value
overrides don't work due "kernel and drivers" order, the kernel defines
the original symbol with the default and then it cannot be overridden.

Move the kernel symbol override to the original definition to make it
affect. Else any sample that uses nRF70 but doesn't enable WPA
supplicant ends up with 1024 and crashes.

Signed-off-by: Chaitanya Tata <Chaitanya.Tata@nordicsemi.no>