PCI: designware: Use exact access size in dw_pcie_cfg_read()

dw_pcie_cfg_write() uses the exact 8-, 16-, or 32-bit access size
requested, but dw_pcie_cfg_read() previously performed a 32-bit read and
masked out the bits requested.

Use the exact access size in dw_pcie_cfg_read().  For example, if we want
an 8-bit read, use readb() instead of using readl() and masking out the 8
bits we need.  This makes it symmetric with dw_pcie_cfg_write().

[bhelgaas: split into separate patch, set *val = 0 in failure case]
Signed-off-by: Gabriele Paoloni <gabriele.paoloni@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>