mgmt/osdp: Handle errors in uart_fifo_read()

Since 581c20e2 ("drivers: uart: Cleanup not supported API handling")
uart_fifo_read() returns -ve on errors. Correct the check on its return
value to work with this new API.

CID: 240662
Fixes: #39840
Signed-off-by: Siddharth Chandrasekaran <sidcha.dev@gmail.com>