media: lirc: when transmitting scancodes, block until transmit is done

The semantics for lirc IR transmit with raw IR is that the write call
should block until the IR is transmitted. Some drivers have no idea
when this actually is (e.g. mceusb), so there is a wait.

This is useful for userspace, as it might want to send a IR button press,
a gap of a predefined number of milliseconds, and then send a repeat
message.

It turns out that for transmitting scancodes this feature is even more
useful, as user space has no idea how long the IR is. So, maintain
the existing semantics for IR scancode transmit.

Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>