ACPI / PMIC: xpower: fix IOSF_MBI dependency

We still get a link failure with IOSF_MBI=m when the xpower driver
is built-in:

drivers/acpi/pmic/intel_pmic_xpower.o: In function `intel_xpower_pmic_update_power':
intel_pmic_xpower.c:(.text+0x4f2): undefined reference to `iosf_mbi_block_punit_i2c_access'
intel_pmic_xpower.c:(.text+0x5e2): undefined reference to `iosf_mbi_unblock_punit_i2c_access'

This makes the dependency stronger, so we can only build when IOSF_MBI
is built-in.

Fixes: 6a9b593d (ACPI / PMIC: xpower: Add depends on IOSF_MBI to Kconfig entry)
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>