mtd: onenand: make onenand_scan_bbt() static

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: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>