phy: rcar-gen3-usb2: use enum phy_mode in the role_store()

This patch modifies the role_store() to use "enum phy_mode" instead
of the local "bool" for host/device mode selection.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>