firmware: read firmware size using i_size_read()

There is no need to read attr because inode structure contains size
of the file. Use i_size_read() instead.

Signed-off-by: Dmitry Kasatkin <d.kasatkin@samsung.com>
Acked-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>