serial: sh-sci: Always set TIOCM_CTS in .get_mctrl() callback
Documentation/serial/driver clearly states:
If the port does not support CTS, DCD or DSR, the driver should
indicate that the signal is permanently active.
Hence always set TIOCM_CTS, as we currently don't look at the CTS
hardware line state at all.
FWIW, this fixes the transmit path when hardware-assisted flow control
is enabled, and userspace enables CRTSCTS.
The receive path is still broken, as RTS is never asserted.