PCI: keystone: Cleanup set_dbi_mode() and get_dbi_mode()

No functional change. Use BIT() macro for DBI_CS2 and cleanup
set_dbi_mode() and get_dbi_mode().

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>