[media] ttusb_dec: avoid the risk of go past buffer

Fixes this smatch warning:
	drivers/media/usb/ttusb-dec/ttusb_dec.c:243 ttusb_dec_handle_irq() error: buffer overflow 'rc_keys' 26 <= 126

As the RC keys should be enabled previously, via:
	set_bit(rc_keys[i], input_dev->keybit);

It wouldn't go past the buffer in practice. Yet, as bad
things may happen when going past buffer, it doesn't hurt adding
a check here.

While here, fix CodingStyle issues on the routine.

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