USB: usbtmc: Use explicit unsigned type for input buffer instead of char*

Silences compiler warning about comparison with 0x80, and type now matches the
corresponding _bulk_out function.

drivers/usb/class/usbtmc.c: In function ‘usbtmc_ioctl_abort_bulk_in’:
drivers/usb/class/usbtmc.c:163: warning: comparison is always false due to limited range of data type

Signed-off-by: Chris Malley <mail@chrismalley.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>