drivers: flash: stm32h7: ignore STRBERR during flash write

When application code runs from M4 (resides on lower 896KiB of
the internal flash of bank2) and it does write to a block in
the upper 128KiB, write fails sometime (for example failed
1 out of 10 block write based on the tests) due to STRBERR.
As per technical reference manual section 4.7.4 of
STM32H745/755 SoC, application may ignore this error and
continue with write. So check for the flag and return okay
status for write(). Based on our tests, the application write
succeeds even when this error happens and match with document
description. So igoring this error flag is okay during write.

Signed-off-by: Murali Karicheri <murali.karicheri@sandc.com>