In each case, the function has an unsigned return type, but returns a
negative constant to indicate an error condition. For move_block_from_dma,
there is only one call and the return value is dropped, so it need not be
unsigned. For labpc_eeprom_write, there is only one call and the result is
stored in a signed variable, so again the unsigned return type is not
necessary.
A sematic match that finds this problem is as follows:
(http://coccinelle.lip6.fr/)