mtd: cfi_cmdset_0002: Initialize datum before calling map_word_load_partial

In do_otp_write we must initialize the variable datum before calling
map_word_load_partial. Otherwise the upper bits of datum may be undefined,
which later causes problems in chip_good called by do_write_oneword.

Signed-off-by: Christian Riesch <christian.riesch@omicron.at>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>