mtd: nand: fsmc: Remove BUG macros

Remove the BUG macros and return with error (if possible) instead.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>