clk: bcm2835: fix check of error code returned by devm_ioremap_resource()

The change fixes potential oops while accessing iomem on invalid
address, if devm_ioremap_resource() fails due to some reason.

The devm_ioremap_resource() function returns ERR_PTR() and never
returns NULL, which makes useless a following check for NULL.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Fixes: 5e63dcc7 ("clk: bcm2835: Add a driver for the auxiliary peripheral clock gates")
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>