drivers: flash: Fix Nios-II QSPI flash Coverity issue

Fix Coverity "Memory - corruptions (ARRAY_VS_SINGLETON)"
issue by type casting word_to_write to u8_t pointer and
adding a length check before memcpy operation.

Coverity CID: 182779

Signed-off-by: Ramakrishna Pallala <ramakrishna.pallala@intel.com>