phy: cpcap-usb: Fix error path when no host driver is loaded

If musb_mailbox() returns an error, we must still continue to finish
configuring the phy.

Otherwise the phy state may end up only half initialized, and this can
cause the debug serial console to stop working. And this will happen if the
usb driver musb controller is not loaded.

Let's fix the issue by adding helper for cpcap_usb_try_musb_mailbox().

Fixes: 6d6ce40f ("phy: cpcap-usb: Add CPCAP PMIC USB support")
Cc: Merlijn Wajer <merlijn@wizzup.org>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>