tests: drivers: udc: fix uninitialized variable and thread

Variable to get MPS supported by the controller was used
uninitialized. This caused the test to fail depending on
the memory state.

test_udc_device_get has to be called only once, at the very
beginning. This was overlooked during migration to new test API.

Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>