bna: fix error handling of bnad_get_flash_partition_by_offset()

The current error handling doesn't work because we flash_part is a u32
so the checks for negative error codes don't work.  I considered making
things signed but I don't know the hardware enough to say if that's a
problem.  Really, we don't use the error codes so just returning zero
for all problems is fine.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>