platform: msm: qpnp-revid: replace #ifdef with #if IS_ENABLED()

Replace #ifdef CONFIG_QPNP_REVID with #if
IS_ENABLED(CONFIG_QPNP_REVID).  This fixes a function
redefinition error when compiling the qpnp-revid driver as
a module with CONFIG_QPNP_REVID=m in a defconfig file.

Change-Id: Ie1907010b8bbcd13fdc301f5647af9e081f6088a
Signed-off-by: David Collins <collinsd@codeaurora.org>