gard: Add usage message for -p
gard: Add usage message for -p

The -p argument only really makes sense when -f is specified. Print an
actual error message rather than just the usage blob.

Signed-off-by: Oliver O'Halloran <3dfaff8fa6ae977f042064a112a6aac576279b9b@gmail.com>
Signed-off-by: Stewart Smith <ec31ab75ddf977353c8f660f92ea8b23f64aef25@linux.vnet.ibm.com>

stewart-ibm/skiboot,qemu/skiboot,shenki/skiboot,legoater/skiboot,legoater/skiboot,qemu/skiboot,shenki/skiboot,qemu/skiboot,stewart-ibm/skiboot,shenki/skiboot,legoater/skiboot,open-power/skiboot,shenki/skiboot,open-power/skiboot,open-power/skiboot,legoater/skiboot,shenki/skiboot,qemu/skiboot,open-power/skiboot,stewart-ibm/skiboot,legoater/skiboot,qemu/skiboot,open-power/skiboot