sc16is7xx: fix RTS/CTS implementation and add reading CTS

[Pascal JEAN]

Hi Greg,

Here are explanations of the two patches:

0001-sc16is7xx-fix-RTS-CTS-implementation.patch:
RTS/CTS handshaking for sc16is7xx driver does not work, no character is sent 
regardless of the state of CTS.

This test was done on a raspberry pi 2:

1- sc16is7xx_set_baud() overwrites the flow control configuration bits in EFR. 
Writing in EFR has been replaced by an update.
2- As noted in the file Documentation/serial/driver, 
get_mctrl() should return an active state for unsupported entries.

0002-sc16is7xx-add-reading-CTS.patch:
This patch adds the possibility to read the actual status of the CTS input when
 RTS/CTS handshaking is not activated.

After those changes everything works.

Thank you for your hard work.
Have a nice day.
Pascal