IB/qib: Fix undefined symbol error when CONFIG_PCI_MSI=n

This patch fixes a compile error saying qib_init_iba6120_funcs() is
undefined when CONFIG_PCI_MSI is not defined.  Thanks to Randy Dunlap
<randy.dunlap@oracle.com> for finding this and suggesting the fix.

Signed-off-by: Ralph Campbell <ralph.campbell@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>