arm64: macro whitespace fixes

While working on arch/arm64/include/asm/uaccess.h, I noticed
that one macro within this header is made harder to read because it
violates a coding style rule: space is missing after comma.

Fix it up.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Will Deacon <will.deacon@arm.com>