MIPS: TXx9: 7segled: add missing put_device call

This is required so that we give up the last reference to the device.

Also, add a new tx_7segled_release function which will be called after the
put_device to ensure that device is kfree'd.

[ralf@linux-mips.org: Reformat to Linux coding style and make
tx_7segled_release static.]

Signed-off-by: Levente Kurusa <levex@linux.com>
Cc: LKML <linux-kernel@vger.kernel.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6260/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>