nand.h: Add back 'ops' to `struct nand_chip'

Partial revert of http://git.infradead.org/mtd-2.6.git/blobdiff/4180f24a7bff3aa7978e3785d0edd5dcc4af9049..4a89ff885ff9f64ea62669100766e10e4e257c6e:/include/linux/mtd/nand.h