usbtmc: remove redundant braces

There is a few cases where braces are not needed. This patch removes
unnecessary '& 255' pieces as well when lvalue type is u8.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>