DSI has separate TX and RX fifos. However, the current code only has one
field where the fifo size is stored, and the code for both TX and RX
config write to the same field. This has not caused issues, as we've
been using the same fifo sizes.

Fix this bug by creating separate fields for TX and RX fifo sizes.
Signed-off-by: NTomi Valkeinen <tomi.valkeinen@ti.com>