remoteproc: st: fix check of syscon_regmap_lookup_by_phandle() return value

syscon_regmap_lookup_by_phandle() returns either a valid pointer to
struct regmap or ERR_PTR() error value, check for NULL is invalid and
on error path may lead to oops, the change corrects the check.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>