net: lwm2m: Direct firmware_transfer() call instead of work-item

The LwM2M firmware pull object no longer uses the system workqueue
to execute firmware_transfer(), but directly executes it itself.
Previously, the workqueue would be blocked because firmware_transfer()
indirectly calls a blocking connect(). This would lead to problems
with e.g. modem drivers that use UART to interface with the modem
hardware, as some UART drivers use the workqueue.

Fixes #31053.

Signed-off-by: Maik Vermeulen <maik.vermeulen@innotractor.com>