usb: f_cdev: Handle the serial state properly

Currently when dtr high is send by the host, dsr high should be
sent by the device by performing a term io to acm conversion.
But if dtr high is received twice, serial state which is converted
to acm signal is again converted, leading to serial state becoming
null. Fix this by clearing ACM bits from previous conversion before
performing uart to acm conversion.

Change-Id: Iaad3f200599f6bd4224db0c3987420cc022b34ef
Signed-off-by: Rohith Kollalsi <quic_rkollals@quicinc.com>