regulator: stm32-pwr: fix of_iomap leak

In the Linux kernel, the following vulnerability has been resolved:

regulator: stm32-pwr: fix of_iomap leak

Smatch reports:
drivers/regulator/stm32-pwr.c:166 stm32_pwr_regulator_probe() warn:
'base' from of_iomap() not released on lines: 151,166.

In stm32_pwr_regulator_probe(), base is not released
when devm_kzalloc() fails to allocate memory or
devm_regulator_register() fails to register a new regulator device,
which may cause a leak.

To fix this issue, replace of_iomap() with
devm_platform_ioremap_resource(). devm_platform_ioremap_resource()
is a specialized function for platform devices.
It allows 'base' to be automatically released whether the probe
function succeeds or fails.

Besides, use IS_ERR(base) instead of !base
as the return value of devm_platform_ioremap_resource()
can either be a pointer to the remapped memory or
an ERR_PTR() encoded error code if the operation fails.