efi_loader: search all possible disk partitions

When searching for partitions don't stop if a partition is not present
for a given partition number as there may be valid partitions after.

Search for up to MAX_SEARCH_PARTITIONS matching the other callers of
part_get_info().

This allows OpenBSD to boot via the efi_loader on rpi_3 again after
changes made after U-Boot 2017.09.  With MBR partitioning OpenBSD will
by default use the fourth partition for the 0xA6 (OpenBSD) partition.

Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>