Like was done in commit 17799359
("mtd: nand_bbt: make nand_scan_bbt() static") for the NAND code, this
commit makes the onenand_scan_bbt() function static in the OneNAND
code, since it is only used in onenand_bbt.c itself.

Consequently, the EXPORT_SYMBOL() and declaration in bbm.h are also
removed.
Signed-off-by: NThomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: NBoris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: NBrian Norris <computersforpeace@gmail.com>