samples: mgmt: mcumgr: smp_svr: Fix stack overflow with bt

Fixes an issue with a possible stack overflow when using the
Bluetooth transport for large mcumgr transfer, the issue was caused
by moving to a dedicated workqueue but not moving the enlarged
system workqueue overlay to the new smp workqueue.

Fixes #51640

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
(cherry picked from commit ff624a18)