USB: assign instead of equal in usbtmc.c

Assign operator instead of equality test in the usbtmc_ioctl_abort_bulk_in() function.

Signed-off-by: Maxim A. Nikulin <M.A.Nikulin@gmail.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>