mtd: nand: ifc: update bufnum mask for ver >= 2.0.0

Bufnum mask is used to calculate page position in the internal SRAM.

As IFC version 2.0.0 has 16KB of internal SRAM as compared to older
versions which had 8KB. Hence bufnum mask needs to be updated.

Signed-off-by: Jagdish Gediya <jagdish.gediya@nxp.com>
Signed-off-by: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>