modbus-rtu: don't use O_EXCL when opening the device

[mhei]

Quote from 'man 2 open': -snip- ... In general, the behavior of O_EXCL is undefined if it is used without O_CREAT. ... -snap-

Since we don't create the device file here - we just want to open (hopefully) existing ones - let's simply drop this flag to be on the safe side.