USB: serial: mos7840: remove set but not used variables 'number, serial'

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

drivers/usb/serial/mos7840.c: In function 'mos7840_send_cmd_write_baud_rate':
drivers/usb/serial/mos7840.c:1584:16: warning:
 variable 'number' set but not used [-Wunused-but-set-variable]

drivers/usb/serial/mos7840.c: In function 'mos7840_change_port_settings':
drivers/usb/serial/mos7840.c:1695:21: warning:
 variable 'serial' set but not used [-Wunused-but-set-variable]

'number' never used since introduction in commit 3f542974 ("USB: Moschip
7840 USB-Serial Driver")

'serial' not used since commit 5833041f ("USB: serial: remove unnecessary
reinitialisations of urb->dev")

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