sc16is7xx: Set AUTOCTS and AUTORTS bits

sc16is7xx: Set AUTOCTS and AUTORTS bits

Let serial core know that the chip automatically handles RTS/CTS signal.
This elimines completely unnecessary I2C/SPI bus traffic.

Cease reading from RX FIFO (by disabling RDI interrupt) when throttled.
Eventually the FIFO will fill up and the device will drive RTS output
inactive. Unthrottle by enabling back RDI interrupt.

Indirectly controlling RTS via RX FIFO state seems to be the only option
because RTS bit is ignored when hardware flow control is enabled.

Signed-off-by: Tomasz Moń <tomasz.mon@camlingroup.com>
Link: https://lore.kernel.org/r/20220301060332.2561851-4-tomasz.mon@camlingroup.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>