serial: ns16550: Call usbtty_poll only in non-SPL build

Having both USBTTY and CONFIG_SERIAL_MULTI enabled in SPL, the
usbtty.c file is protected in Makefile to not be compiled into
the SPL. Yet, the ns16550 serial driver does not contain such
protection. Add it to avoid missing symbol error.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Tom Rini <trini@ti.com>