optee: Use bitmap_free() to free bitmap

kfree() and bitmap_free() are the same. But using the latter is more
consistent when freeing memory allocated with bitmap_zalloc().

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>