usb: phy: remove redundant store to variable var after & operation

There is no need to store the result of the & operation back to the
variable var. The store is redundant, replace &= with just &.

Cleans up clang scan warning:
drivers/usb/phy/phy-keystone.c:62:5: warning: Although the value stored to
'val' is used in the enclosing expression, the value is never actually read
from 'val' [deadcode.DeadStores]
Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Link: https://lore.kernel.org/r/20220715225657.353828-1-colin.i.king@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/phy/phy-keystone.c

@@ -59,7 +59,7 @@ static void keystone_usbphy_shutdown(struct usb_phy *phy)
 
 	val  = keystone_usbphy_readl(k_phy->phy_ctrl, USB_PHY_CTL_CLOCK);
 	keystone_usbphy_writel(k_phy->phy_ctrl, USB_PHY_CTL_CLOCK,
-				val &= ~PHY_REF_SSP_EN);
+				val & ~PHY_REF_SSP_EN);
 }
 
 static int keystone_usbphy_probe(struct platform_device *pdev)