tty: serial: qcom_geni_serial: Move loopback support to TIOCM_LOOP

Remove code from the driver that create and maintain loopback sysfs node.
Instead use the ioctl TIOCMSET with TIOCM_LOOP argument to set HW to
loopback mode.

Signed-off-by: Akash Asthana <akashast@codeaurora.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/1578321905-25843-3-git-send-email-akashast@codeaurora.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>