the return type of function _read_ in pstore is size_t,
but in the callback function of _read_, the logic doesn't
consider it too much, which means if negative value (assuming
error here) is returned, it will be converted to positive because
of type casting. ssize_t is enough for this function.
Signed-off-by: NChen Gong <gong.chen@linux.intel.com>
Signed-off-by: NTony Luck <tony.luck@intel.com>