eth: fbnic: Create fw_log file in DebugFS

Allow reading the firmware log in DebugFS by accessing the fw_log file.
Buffer is read while a spinlock is acquired.

Signed-off-by: Lee Trager <lee@trager.us>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20250702192207.697368-7-lee@trager.us


Signed-off-by: Jakub Kicinski <kuba@kernel.org>