Kconfig.nrf: Update CONFIG_MCUMGR_TRANSPORT_NETBUF_SIZE default

Update the condition for default value to align with new name of the
Kconfig option.

Jira: NCSDK-20655

Signed-off-by: Marek Pieta <Marek.Pieta@nordicsemi.no>

Kconfig.nrf

@@ -74,7 +74,7 @@ config PRIVILEGED_STACK_SIZE
 # nRF Connect SDK needs larger mcumgr buffers when supporting multi-image DFU
 # or when the Bluetooth Reassembly feature is enabled.
 config MCUMGR_TRANSPORT_NETBUF_SIZE
-	default 2475 if MCUMGR_SMP_REASSEMBLY_BT
+	default 2475 if MCUMGR_TRANSPORT_BT_REASSEMBLY
 	default 1024 if UPDATEABLE_IMAGE_NUMBER > 1
 
 # When using HCI on the nRF5340 we need a larger command buffer.