flash/nor/core: fix double-free crash with 'virtual' flash banks

flash/nor/core: fix double-free crash with 'virtual' flash banks

flash_bank structure of 'virtual' flash driver is a full copy of
the master flash_bank structure including bank->sectors and
bank->prot_blocks pointers. These pointers point to memory
locations allocated by the master driver and thus master driver
is responsible for deallocating them.
Do not free bank->sectors and bank->prot_blocks of 'virtual'
driver since they were already released by master flash driver.

Change-Id: I01f373d4adb3fc79e2724964926b9276442c5c52
Signed-off-by: Bohdan Tymkiv <bhdt@cypress.com>
Reviewed-on: http://openocd.zylin.com/4504
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>