[media] mantis: get rid of warning: no previous prototype

drivers/media/pci/mantis/mantis_input.c:107:5: warning: no previous prototype for 'mantis_input_init' [-Wmissing-prototypes]
drivers/media/pci/mantis/mantis_input.c:153:5: warning: no previous prototype for 'mantis_exit' [-Wmissing-prototypes]
drivers/media/pci/mantis/mantis_uart.c:64:5: warning: no previous prototype for 'mantis_uart_read' [-Wmissing-prototypes]
drivers/media/pci/mantis/mantis_vp1033.c:118:5: warning: no previous prototype for 'lgtdqcs001f_set_symbol_rate' [-Wmissing-prototypes]
drivers/media/pci/mantis/mantis_vp1033.c:86:5: warning: no previous prototype for 'lgtdqcs001f_tuner_set' [-Wmissing-prototypes]

Note: there is already a mantis_exit at the mantis driver. So,
this should be renamed to mantis_input_exit.

The mantis_input code is currently unused, as it doesn't implement the RC API
right.

Patches for it are sent for discussions, but were never
accepted. So, while this is not fixed, the entire code inside
mantis_input can simply be commented.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>