atm: Replace custom isprint() with generic analogue

Custom isprint() definition may collide with one form ctype.h.
In order to avoid this, replace it with a functional analogue
which is isascii() && isprint() in this case.

First appearance of the code is in the commit 636b38438001
("Import 2.3.43").

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>