When MUSB is a loadable module, we get a link error for a built-in
CPCAP driver:

drivers/phy/built-in.o: In function `cpcap_usb_phy_remove':
phy-cpcap-usb.c:(.text+0xed9): undefined reference to `musb_mailbox'

This adds a Kconfig dependency to prevent this broken configuration,
enforcing that CPCAP can only be a module when MUSB is also a module.

Fixes: 68a1f7c9d470 ("phy: cpcap-usb: Add CPCAP PMIC USB support")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>