soc: qcom: spm: fix building with CONFIG_REGULATOR=n

soc: qcom: spm: fix building with CONFIG_REGULATOR=n

The newly added code causes a build failure when -Werror is set:

drivers/soc/qcom/spm.c:388:12: error: 'spm_get_cpu' defined but not used [-Werror=unused-function]

Remove the #ifdef and instead use an IS_ENABLED() check that lets the
compiler perform dead code elimination instead of the preprocessor.

Fixes: 6496dba142f4 ("soc: qcom: spm: add support for voltage regulator")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20240221154457.2007420-1-arnd@kernel.org
Signed-off-by: Bjorn Andersson <andersson@kernel.org>