mtd: rawnand: au1550nd: Fix au_read_buf16() prototype

Commit 7e534323 ("mtd: rawnand: Pass a nand_chip object to
chip->read_xxx() hooks") modified the prototype of the struct nand_chip
read_buf function pointer. In the au1550nd driver we have 2
implementations of read_buf. The previously mentioned commit modified
the au_read_buf() implementation to match the function pointer, but not
au_read_buf16(). This results in a compiler warning for MIPS
db1xxx_defconfig builds:

  drivers/mtd/nand/raw/au1550nd.c:443:57:
    warning: pointer type mismatch in conditional expression

Fix this by updating the prototype of au_read_buf16() to take a struct
nand_chip pointer as its first argument, as is expected after commit
7e534323 ("mtd: rawnand: Pass a nand_chip object to chip->read_xxx()
hooks").

Note that this shouldn't have caused any functional issues at runtime,
since the offset of the struct mtd_info within struct nand_chip is 0
making mtd_to_nand() effectively a type-cast.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Fixes: 7e534323 ("mtd: rawnand: Pass a nand_chip object to chip->read_xxx() hooks")
Cc: stable@vger.kernel.org # v4.20+
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>