mdm9615: Fix bug which turns off hardware gating for usb_hs1_pclk

LK was inadvertently overwriting the settings for the clock
control register usb_hs1_pclk when it does a write to enable
the clock.

Changing this behavior to read-modify-write leaves the
SBL settings intact.
diff --git a/platform/mdm9x15/acpuclock.c b/platform/mdm9x15/acpuclock.c
index d29055b..6096731 100644
--- a/platform/mdm9x15/acpuclock.c
+++ b/platform/mdm9x15/acpuclock.c
@@ -95,6 +95,8 @@
 
 void hsusb_clock_init(void)
 {
+	uint32_t reg;
+
 	pll8_enable();
 
 	/* Setup USB HS1 System clock - 60 Mhz */
@@ -113,7 +115,9 @@
 				USB_HS1_XCVR_FS_CLK_MD);
 
 	/* HS-USB: enable cc_usb_hs1_hs_clk */
-	writel( 0x1 << 4, USB_HS1_HCLK_CTL);
+	reg = readl(USB_HS1_HCLK_CTL);
+	reg |= 0x1 << 4;
+	writel( reg, USB_HS1_HCLK_CTL);
 }
 
 /* Configure UART clock - based on the gsbi id */