sc16is7xx: Fix for hardware flow control

The SC16IS7XX hardware flow control is mishandled by the driver in
a number of ways:

  1. The set_baud method accidentally clears it when setting EFR bit.
  2. Even though hardware flow control is enabled, it isn't indicated
     back to the serial framework.
  3. Applying the flow control clears the EFR bit.
  4. The CTS support is not indicated in the return from
     sc16is7xx_get_mctrl.

Address all of those issues using a mixture of patches found on the
linked pages.

See: https://github.com/raspberrypi/linux/issues/2542
See: https://www.spinics.net/lists/linux-serial/msg21794.html



Signed-off-by: Phil Elwell <phil@raspberrypi.com>