mfiutil: Handle potential ioctl(2) failures in mfi_flash.c

The return value of function 'mfi_dcmd_command' should always be checked for
the potential ioctl(2) failure.

PR: 281158
Pull Request: https://github.com/freebsd/freebsd-src/pull/1403

(cherry picked from commit bac98f86c98421f6153c749b6c77995ef778e2fb)