media: rc: ir-spi: constrain carrier frequency

Carrier frequency is currently unconstrained, allowing the SPI transfer
to be allocated and filled only for it to be later rejected by the SPI
controller since the frequency is too large.

Add a check to constrain the carrier frequency inside
ir_spi_set_tx_carrier().

Also, move the number of bits per pulse to a macro since it is not used
in multiple places.

Signed-off-by: Cosmin Tanislav <demonsingur@gmail.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>