coresight: fix uninitialized variable warning

The compiler thinks that phys_pte could be used uninitialized (if
sg_tbl_num == 0). Fix this.

The specific warning being fixed is:

    drivers/coresight/coresight-tmc.c: In function 'tmc_read':
    drivers/coresight/coresight-tmc.c:1411:14: warning: 'phys_pte' may \
        be used uninitialized in this function [-Wuninitialized]
    error, forbidden warning: coresight-tmc.c:1411

Change-Id: Id26d5ccc8230dc5409c53dc14925ec9067d3dd02
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>