USB: serial: cypress_m8: remove set but not used variable 'iflag'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/usb/serial/cypress_m8.c: In function 'cypress_set_termios':
drivers/usb/serial/cypress_m8.c:866:18: warning:
 variable 'iflag' set but not used [-Wunused-but-set-variable]

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Johan Hovold <johan@kernel.org>