mtd: onenand_base: Mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

This patch fixes the following warning:

drivers/mtd/nand/onenand/onenand_base.c: In function ‘onenand_check_features’:
drivers/mtd/nand/onenand/onenand_base.c:3264:17: warning: this statement may fall through [-Wimplicit-fallthrough=]
   this->options |= ONENAND_HAS_NOP_1;
drivers/mtd/nand/onenand/onenand_base.c:3265:2: note: here
  case ONENAND_DEVICE_DENSITY_4Gb:
  ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enable
-Wimplicit-fallthrough.

Cc: Jonathan Bakker <xc-racer2@live.ca>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>