snp_issue_guest_request() checks the value returned by the hypervisor in
sw_exit_info_2 and returns a different error depending on it.

Convert those checks into a switch-case to make it more readable when
more error values are going to be checked in the future.

No functional changes.
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20230307192449.24732-8-bp@alien8.de