firmware_class: Fix the file size check

We expect to read firmware blobs with a single call to kernel_read(),
which returns int.  Therefore the size must be within the range of
int, not long.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>