PCI/switchtec: Fix vep_vector_number ioread width

vep_vector_number is actually a 16 bit register which should be read with
ioread16() instead of ioread32().

Fixes: 080b47de ("MicroSemi Switchtec management interface driver")
Link: https://lore.kernel.org/r/20200106190337.2428-3-logang@deltatee.com


Reported-by: Doug Meyer <dmeyer@gigaio.com>
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>