i3c: dw: Create a generic fifo read function

In a future change we'll want to read from the IBI FIFO too, so turn
dw_i3c_read_rx_fifo() into a generic read with the FIFO register as a
parameter.

Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Link: https://lore.kernel.org/r/827204789583dd86addffb47ecaeab9d67cf95d5.1680161823.git.jk@codeconstruct.com.au
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>