bnxt_en: don't consider building bnxt_tc.o if option not enabled

Instead of zeroing out bnxt_tc.c with a #ifdef foo, instead don't compile
the file when the option is not enabled. Now make and the preprocessor do
not have to waste time compiling a no-op.

Signed-off-by: Jonathan Toppins <jtoppins@redhat.com>
Acked-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>