USB: option: Remove unused variable

After commit f092c240 ("USB: option:
remove unnecessary and erroneous code") the variable 'serial' becomes
unused, as gcc-4.3.2 points out:

drivers/usb/serial/option.c: In function 'option_instat_callback':
drivers/usb/serial/option.c:834: warning: unused variable 'serial'
drivers/usb/serial/option.c: In function 'option_open':
drivers/usb/serial/option.c:930: warning: unused variable 'serial'

So I removed it.

Signed-off-by: Carlos R. Mafra <crmafra@aei.mpg.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>