mtd: nand: pxa3xx: Add a nice comment to pxa3xx_set_datasize()

Add a comment clarifying the use of pxa3xx_set_datasize() which is only
applicable on data read/write commands (i.e. commands with a data cycle,
such as READID, READ0, STATUS, etc.)

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>