sfxge: do not use DEBUG_FLAGS to enable extra debug checks

DEBUG_FLAGS are set to DEBUG option value when kernel is built.
For example, it is -g in GENERIC config to have debug symbols.
Also DEBUG_FLAGS are used to determine if ctfconvert should keep
debug symbols.
Since we redefined DEBUG_FLAGS, debug symbols were always missing.
ctfconvert complains about it during kernel build.
It is incorrect to append DEBUG_FLAGS, since if DEBUG has no -g (or
similar), we'll have no debug symbols and ctfconvert will complain.
If it incorrect to always have -g in our DEBUG_FLAGS, since debug
symbols presence should be controllable by kernel config.
So, just add disabled by default addition of -DDEBUG=1 to CFLAGS.

Reviewed by: imp
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D2666