USB: otg: remove unused function twl6030_set_phy_clk

Remove the unused function twl6030_set_phy_clk of twl6030-usb.c.

Signed-off-by: Wanlong Gao <gaowanlong@cn.fujitsu.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>