usb: ehci-platform: increase EHCI_MAX_RSTS to 4

Socionext LD11 SoC (arch/arm64/boot/dts/socionext/uniphier-ld11.dtsi)
needs to handle 4 reset lines for EHCI.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>