USB: serial: mos7840: remove set but not used variables 'st, data1, iflag'

USB: serial: mos7840: remove set but not used variables 'st, data1, iflag'

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

drivers/usb/serial/mos7840.c: In function 'mos7840_interrupt_callback':
drivers/usb/serial/mos7840.c:604:14: warning:
 variable 'st' set but not used [-Wunused-but-set-variable]

drivers/usb/serial/mos7840.c: In function 'mos7840_write':
drivers/usb/serial/mos7840.c:1303:17: warning:
 variable 'data1' set but not used [-Wunused-but-set-variable]

drivers/usb/serial/mos7840.c:1700:11: warning:
 variable 'iflag' set but not used [-Wunused-but-set-variable]

They are never used since introduction in commit 3f5429746d91 ("USB:
Moschip 7840 USB-Serial Driver")

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