loader.efi: ResetSystem does not use data with EFI_SUCCESS

The current reboot command in efi/loader/main.c is passing extra data with
ResetSystem, however, UEFI spec 2.6, page 265 does state:

"ResetData is only valid if ResetStatus is something other than EFI_SUCCESS
unless the ResetType is EfiResetPlatformSpecific where a minimum amount of
ResetData is always required."

Therefore we should use DataSize 0 and ResetData NULL - those are two last
arguments for the call.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D10562