mtd: nandsim: Staticize local symbols

These local symbols are used only in this file.
Fix the following sparse warnings:

drivers/mtd/nand/nandsim.c:1436:5: warning: symbol 'do_read_error' was not declared. Should it be static?
drivers/mtd/nand/nandsim.c:1448:6: warning: symbol 'do_bit_flips' was not declared. Should it be static?

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>