[media] rc: set IR_MAX_DURATION to 500 ms

The current definition is weird, and produce lots of sparse
warnings:

	drivers/media/i2c/cx25840/cx25840-ir.c:448 txclk_tx_s_max_pulse_width() warn: impossible condition '(ns > 4294967295) => (0-u32max > u32max)'
	drivers/media/i2c/cx25840/cx25840-ir.c:461 rxclk_rx_s_max_pulse_width() warn: impossible condition '(ns > 4294967295) => (0-u32max > u32max)'
	drivers/media/i2c/cx25840/cx25840-ir.c:706 cx25840_ir_rx_read() warn: impossible condition '(v > 4294967295) => (0-u32max > u32max)'
	drivers/media/pci/ivtv/ivtv-queue.c:145 ivtv_queue_move() error: we previously assumed 'steal' could be null (see line 138)
	drivers/media/rc/streamzap.c:155 sz_push_full_pulse() warn: impossible condition '(rawir.duration > 4294967295) => (0-u32max > u32max)'
	drivers/media/rc/streamzap.c:169 sz_push_full_pulse() warn: impossible condition '(rawir.duration > 4294967295) => (0-u32max > u32max)'
	drivers/media/rc/redrat3.c:325 redrat3_us_to_len() warn: impossible condition '(microsec > 4294967295) => (0-u32max > u32max)'
	drivers/media/rc/redrat3.c:383 redrat3_process_ir_data() warn: impossible condition '(rawir.duration > 4294967295) => (0-u32max > u32max)'
	drivers/media/usb/pvrusb2/pvrusb2-hdw.c:3676 pvr2_send_request_ex() error: we previously assumed 'write_data' could be null (see line 3648)
	drivers/media/usb/pvrusb2/pvrusb2-hdw.c:3829 pvr2_send_request_ex() error: we previously assumed 'read_data' could be null (see line 3649)
	drivers/media/pci/cx23885/cx23888-ir.c:463 txclk_tx_s_max_pulse_width() warn: impossible condition '(ns > 4294967295) => (0-u32max > u32max)'
	drivers/media/pci/cx23885/cx23888-ir.c:476 rxclk_rx_s_max_pulse_width() warn: impossible condition '(ns > 4294967295) => (0-u32max > u32max)'
	drivers/media/pci/cx23885/cx23888-ir.c:696 cx23888_ir_rx_read() warn: impossible condition '(v > 4294967295) => (0-u32max > u32max)'

Use a more realistic value for it.

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