efi_loader: use EFI_EXIT in efi_riscv_get_boot_hartid

After calling EFI_ENTRY we have to call EFI_EXIT before returning.

Add a missing EFI_EXIT().

Fixes: 1ccf8716 ("efi_loader: Enable RISCV_EFI_BOOT_PROTOCOL support")
Reported-by: Dave Jones <dave.jones@canonical.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Sunil V L <sunilvl@ventanamicro.com>