kbuild: deb-pkg: add --source-option=-sP

If the source package fails to build, ../linux.orig is left over.
In the next run of 'make deb-pkg', you will get the following error:

  dpkg-source: error: orig directory 'linux.orig' already exists, not overwriting, giving up; use -sA, -sK or -sP to override

You can manually remove ../linux.orig, but it is annoying.

Pass -sP down to dpkg-source.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>