xtensa: macro whitespace fixes

While working on arch/xtensa/include/asm/uaccess.h, I noticed
that some macros within this header are made harder to read because they
violate a coding style rule: space is missing after comma.

Fix it up.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Max Filippov <jcmvbkbc@gmail.com>