pwm: brcmstb: Fix check of devm_ioremap_resource() return code

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: 3a9f5957 ("pwm: Add Broadcom BCM7038 PWM controller support")
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>