modem: ubx: fix incoming byte processing

`ret` holds the amount of bytes received from the modem. However during
processing of the bytes its value is overwritten by the return value of
`modem_ubx_process_received_byte`, in practice discarding all but the
first byte read.

To prevent this, store the length in a separate variable.

Signed-off-by: Anders T. Akre <anders@akre.io>