firewire: add read_csr_reg driver callback

To prepare for the following additions of more OHCI-implemented CSR
registers, replace the get_cycle_time driver callback with a generic
CSR register callback.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
diff --git a/drivers/firewire/core-transaction.c b/drivers/firewire/core-transaction.c
index 6971400..a4d8109 100644
--- a/drivers/firewire/core-transaction.c
+++ b/drivers/firewire/core-transaction.c
@@ -1025,7 +1025,8 @@
 
 	case CSR_CYCLE_TIME:
 		if (TCODE_IS_READ_REQUEST(tcode) && length == 4)
-			*data = cpu_to_be32(card->driver->get_cycle_time(card));
+			*data = cpu_to_be32(card->driver->
+					read_csr_reg(card, CSR_CYCLE_TIME));
 		else
 			rcode = RCODE_TYPE_ERROR;
 		break;