spdx30_tasks.py: switch from exists to isfile checking debugsrc

spdx30_tasks.py: switch from exists to isfile checking debugsrc

Same change as previously made to the create-spdx-2.2.bbclass,
while debugsrc is almost always a file (or link), there are apparently
cases where a directory could be returned from the dwarfsrcfiles
processing.  When this happens, the hashing fails and an error results
when building the SPDX documents.

Signed-off-by: Mark Hatle <mark.hatle@amd.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>