powerpc/xmon: use isspace/isxdigit/isalnum from linux/ctype.h

isxdigit() macro definition is the same.

isalnum() from linux/ctype.h will accept additional latin non-ASCII
characters. This is harmless since this macro is used in scanhex() which
parses user input.

isspace() from linux/ctype.h will accept vertical tab and form feed but
not NULL. The use of this macro is modified to accept NULL as
well. Additional characters are harmless since this macro is also only
used in scanhex().

Signed-off-by: Vincent Bernat <vincent@bernat.im>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>