jz4740: SD clock sequencing changes.

* Don't stop clock before switching speeds
 * Don't stop clock prior to transactions
 * Stop clock at the end of transactions

Will result in slightly better performance and some power saving when
we're not actively using the SD peripheral.

Change-Id: I1c82476cad97137b1469900645ecf7bb0887119a

firmware/target/mips/ingenic_jz47xx/ata-sd-jz4740.c

@@ -623,8 +623,6 @@ static void jz_sd_set_clock(unsigned int rate)
 {
     int clkrt;
 
-    jz_sd_stop_clock();
-
     /* select clock source from CPM */
     cpm_select_msc_clk(rate);
 
@@ -676,9 +674,6 @@ static int jz_sd_exec_cmd(struct sd_request *request)
         }
     }
 
-    /* stop clock */
-    jz_sd_stop_clock();
-
     /* mask all interrupts */
     //REG_MSC_IMASK = 0xffff;
     /* clear status */
@@ -889,6 +884,7 @@ static int jz_sd_exec_cmd(struct sd_request *request)
     }
 
     /* Command completed */
+    jz_sd_stop_clock();  /* Stop SD clock since we're done */
 
     return SD_NO_ERROR;    /* return successfully */
 }