misc: sram: cleanup the code

Since the devm_gen_pool_create() is used, so the gen_pool_destroy()
here is redundant.

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>