firmware_class: Return specific errors from file read

firmware_class: Return specific errors from file read

Currently several failure cases are not distinguished and are
incorrectly reported as -EINVAL or -ENOENT.

Change fw_file_size() to return an error code on failure and
adjust fw_read_file_contents() and fw_get_filesystem_firmware()

Change _request_firmware() to return the error code from
fw_get_filesystem_firmware() if CONFIG_FW_LOADER_USER_HELPER is not
enabled.  (If it is enabled and also fails, unfortunately we can't
tell why.)

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Gbp-Pq: Topic bugfix/all
Gbp-Pq: Name firmware_class-return-specific-errors-from-file-read.patch