wncm14a2a: fix armclang compiler warnings with is*() functions

We get compile warnings of the form:

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 (and the other is* functions) return an int, change check
to an explicit test against the return value.

Signed-off-by: Kumar Gala <kumar.gala@intel.com>