usb: Remove references to non-existent PLAT_S5P symbol

The PLAT_S5P Kconfig symbol was removed in commit d78c16cc
("ARM: SAMSUNG: Remove remaining legacy code"). There are still
some references left, fix that by replacing them with ARCH_S5PV210.

Fixes: d78c16cc ("ARM: SAMSUNG: Remove remaining legacy code")
Reported-by: Paul Bolle <pebolle@tiscali.nl>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>