mtd: nand: pxa3xx: Move the data buffer clean to prepare_start_command()

To allow future support of multiple page reading/writing, move the data
buffer clean out of prepare_set_command().

This is done to prevent the data buffer from being cleaned on every command
preparation, when a multiple command sequence is implemented to read/write
pages larger than the FIFO size (2 KiB).

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