USB: serial: ftdi_sio: remove redundant assignment to variable cflag

The variable cflag is being assigned a value that is not being read
afterwards, it is being re-assigned later on. The assignment is
redundant and can be removed.

Cleans up clang scan warning:
drivers/usb/serial/ftdi_sio.c:2613:15: warning: Value stored to 'cflag'
during its initialization is never read [deadcode.DeadStores]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Johan Hovold <johan@kernel.org>