drivers: flash: bug fix stm32 ospi flash erase

This commit fixes a bug in the STM32 ospi flash driver when attempting
to erase an area that spans more than one erase sector.
Without this fix, only the first sector is actually erased, the rest
silently fails the erase.
Issue is that the write enable latch command is only sent for the first
erase command.

Signed-off-by: Brian Juel Folkmann <bju@trackunit.com>