PCI: dwc: all: Modify dbi accessors to access data of 4/2/1 bytes

Previously dbi accessors can be used to access data of size 4 bytes. But
there might be situations (like accessing MSI_MESSAGE_CONTROL in order to
set/get the number of required MSI interrupts in EP mode) where dbi
accessors must be used to access data of size 2. This is in preparation
for adding endpoint mode support to designware driver.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Niklas Cassel <niklas.cassel@axis.com>
Cc: Jingoo Han <jingoohan1@gmail.com>
Cc: Joao Pinto <Joao.Pinto@synopsys.com>