nand: fix nand torture to use changed mtd api

The mtd subsystem deprecated and renamed the direct use of the mtd_info
struct's functionpointers. Instead the corresponding mtd_xxx function
should be used.

See also:
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=3c3c10bba1e4ccb75b41442e45c1a072f6cded19



Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>