mtd: nand: brcmnand: Check flash #WP pin status before nand erase/program

On brcmnand controller v6.x and v7.x, the #WP pin is controlled through
the NAND_WP bit in CS_SELECT register.

The driver currently assumes that toggling the #WP pin is
instantaneously enabling/disabling write-protection, but it actually
takes some time to propagate the new state to the internal NAND chip
logic. This behavior is sometime causing data corruptions when an
erase/program operation is executed before write-protection has really
been disabled.

Fixes: 27c5b17c

 ("mtd: nand: add NAND driver "library" for Broadcom STB NAND controller")
Signed-off-by: Kamal Dasu <kdasu.kdev@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>