BRCMNAND: Fix reporting of uncorrectable errors on subpages during page read

Previously, a subpage with an uncorrectable error followed by a subpage
with a correctable error would return an erroneous correctable status.

Signed-off-by: Joel Peshkin <joel.peshkin@broadcom.com>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>