x86/bugs: Use early_param() for spectre_v2_user

Most of the mitigations in bugs.c use early_param() to parse their command
line options.  Modify spectre_v2_user to use early_param() for consistency.

Remove spec_v2_user_print_cond() because informing a user about their
cmdline choice isn't very interesting and the chosen mitigation is already
printed in spectre_v2_user_update_mitigation().

Signed-off-by: David Kaplan <david.kaplan@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Link: https://lore.kernel.org/r/20250819192200.2003074-2-david.kaplan@amd.com