IR: add initial IR transmit support

To enable IR transmit support in the mceusb driver, I need to add just
three callbacks to achieve feature-parity with IR transmit using the old
lirc_mceusb driver. This series adds those callbacks to ir-core's
ir_dev_props struct, then wires them up for the mceusb driver.

At the moment, we have no "native" interface for transmitting IR, but a
later patchset to add "legacy" lirc device interface support provides a
means to transmit IR using this ir-core implementation.

[PATCH 1/2] IR: add tx callbacks to ir-core
[PATCH 2/2] IR/mceusb: add and wire up tx callback functions

Patches are against the linuxtv staging/rc tree, but will probably apply
elsewhere as well.

-- 
Jarod Wilson
jarod@redhat.com