armclang: fix compiler warnings with isprint()

We get compile warnings of the form:

drivers/console/uart_console.c:508:8: error: converting the result of
'<<' to a boolean; did you mean
'((__aeabi_ctype_table_ + 1)[(byte)] << 28) != 0'?
 [-Werror,-Wint-in-bool-context]
                if (!isprint(byte)) {
                     ^

Since isprint returns an int, change check to an explicit test against
the return value.

Signed-off-by: Kumar Gala <kumar.gala@intel.com>
(cherry picked from commit 831bd2f8)