rsu: Don't modify read-only firmware block.

The firmware header loaded into an rsu(4) device has to be customized
to reflect device settings. The driver was overwriting the header
from the shared firmware image before sending it to the device. If
two devices attached at the same time with different settings, one
device could potentially get a corrupted header. The recent changes
in a095390344fb1795c1b118a2f84da8f6a7f254ab exposed this bug in the
form of a panic as the firmware blobs are now marked read-only in
object files and mapped read-only by the kernel.

To avoid the bug, change the driver to allocate a copy of the firmware
header on the stack that is initialized before writing it to the
device.

PR: 252163
Reported by: vidwer+fbsdbugs@gmail.com
Tested by: vidwer+fbsdbugs@gmail.com
Reviewed by: hselasky, bz, emaste
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D27850