[media] mantis: fix error handling

Remove dead code, make goto label names more expressive and add a label
in order to call mantis_dvb_exit if mantis_uart_init fails.

Also make sure that mantis_pci_exit is called if we fail the
mantis_stream_control call and that we call mantis_i2c_exit if
mantis_get_mac fails.

[mchehab@osg.samsung.com: fix merge conflict]
Signed-off-by: Silvan Jegen <s.jegen@gmail.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>