media: rc: clean up leader pulse/space for manchester encoding

The IR rc6 encoder sends the header using manchester encoding using 0
bits, which causes the following:

UBSAN: Undefined behaviour in drivers/media/rc/rc-ir-raw.c:247:6
shift exponent 4294967295 is too large for 64-bit type 'long long unsigned int'

So, allow the leader code to send a pulse and space and remove the unused
pulse_space_start field.

Cc: Antti Seppälä <a.seppala@gmail.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>