mtd: nand: remove 'sndcmd' parameter of 'read_oob/read_oob_raw'

As of [mtd: nand: remove autoincrement 'sndcmd' code], the
NAND_CMD_READ0 command is issued unconditionally.

Thus, read_oob/read_oob_raw's 'sndcmd' argument is no longer needed, as
well as their return code.

Remove the 'sndcmd' parameter, and set the return code to 0.

Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>