em28xx: remove backward compat macro added on a previous fix

commit 50f3beb50abe0cc0228363af804e50e710b3e5b0 fixed em28xx-alsa
locking schema. However, a backport macro was kept.

This patch removes the macro, since it is not needed for the module
compilation against upstream.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>