serial: sprd: Fix iotype

The Spreadtrum UART is accessed with mmio; declare the proper iotype.
Also prevent userspace from assigning any other iotype via
ioctl(TIOCSSERIAL).

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>