eth: fbnic: Create ring buffer for firmware logs

When enabled, firmware may send logs messages which are specific to the
device and not the host. Create a ring buffer to store these messages
which are read by a user through DebugFS. Buffer access is protected by
a spinlock.

Signed-off-by: Lee Trager <lee@trager.us>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20250702192207.697368-4-lee@trager.us


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