network: read_line(): Propagate correct error codes
network: read_line(): Propagate correct error codes

The upper layers expect to see the error code and not just -1 in case on an
error.

Fixes: d09449001032 ("Network backend: Speed up read_line() function on Linux")
Signed-off-by: Lars-Peter Clausen <3318dc5ce3e4fb7c28a0b841b6801c884e1d0896@metafoo.de>

gburca/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,gburca/libiio,gburca/libiio,analogdevicesinc/libiio