UBI: make ubi_scan_erase_peb static and rename

The 'ubi_scan_erase_peb()' is used only in scan.c so can be static. Also
re-name it to 'early_erase_peb()' because we tend to use "ubi_" prefix only for
non-static fuction and also because the new name is better.

Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>