PCI: dwc: dra7xx: Help compiler to remove unused code

The dra7xx driver supports both host and ep mode.
When enabling support for only one of the modes, help the compiler
to remove code for the mode that we have not enabled in the driver.

By adding if (!IS_ENABLED(CONFIG_PCI_DRA7XX_HOST)) return -ENODEV;
anything after that statement will get silently dropped by the compiler,
including static functions and structures that are referenced indirectly
from there.

Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Niklas Cassel <niklas.cassel@axis.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>