sof_ipc4_fw_parse_ext_man() can return negative error numbers which is not
correct for the used size_t type.

Change the return value to ssize_t and use the same type where the function
is called.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 73c091a2 ("ASoC: SOF: ipc4-loader: Support for loading external libraries")
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>
Link: https://lore.kernel.org/r/20221025132706.30356-1-peter.ujfalusi@linux.intel.comSigned-off-by: Mark Brown <broonie@kernel.org>