usb: host: Remove optional dependencies on PLAT_S5P

Commit d78c16cc ("ARM: SAMSUNG: Remove remaining legacy code")
removed the Kconfig symbol PLAT_S5P. Remove two optional dependencies on
that symbol from this Kconfig file too.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>