x86/sev: Remove do_early_exception() forward declarations

There's a perfectly fine prototype in the asm/setup.h header. Use it.

No functional changes.

Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211110220731.2396491-8-brijesh.singh@amd.com