rt2x00: Fix incorrect usage of CONFIG_RT2X00_LIB_USB

In device removal routine, usage of "#ifdef CONFIG_RT2X00_LIB_USB"
will not cover the case when it is configured as module. This will
omit the entire if-block which does cleanup of URBs and cancellation
of pending work. Changing the #ifdef to #if IS_ENABLED() to fix it.

Signed-off-by: Vishal Thanki <vishalthanki@gmail.com>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>