mtd: nand: comment update, the DEPLETE1 command has gone

update a comment in nand_command_lp() about specific requirements of
individual commands, the DEPLETE1 command was removed in the past and
the comment no longer applied

Signed-off-by: Gerhard Sittig <gsi@denx.de>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>