[efi] Do not report return status from efi_block_local()

[efi] Do not report return status from efi_block_local()

The return status from efi_block_local() indicates whether or not the
handle is eligible to be assigned a local virtual drive number.  There
will always be several enumerated EFI_BLOCK_IO_PROTOCOL handles that
are not eligible for a local virtual drive number (e.g. the handles
corresponding to partitions, rather than to complete disks), and this
is not an interesting error to report.

Do not report errors from efi_block_local() as the overall error
status for a SAN boot, since doing so would be likely to mask a much
more relevant error from having previously attempted to scan for a
matching filesystem within an eligible block device handle.

Signed-off-by: Michael Brown <mcb30@ipxe.org>