PCI: Drop warning about drivers that don't use pci_set_master()

f41f064c

 ("PCI: Workaround missing pci_set_master in pci drivers") made
pci_enable_bridge() turn on bus mastering if the driver hadn't done so
already.  It also added a warning in this case.  But there's no reason to
warn about it unless it's actually a problem to enable bus mastering here.

This patch drops the warning because I'm not aware of any such problem.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
CC: Paul Bolle <pebolle@tiscali.nl>