Input: lpc32xx_ts - fix warnings caused by enabling unprepared clock

If common clock framework is configured, the driver generates a warning,
which is fixed by this change:

    root@devkit3250:~# cat /dev/input/touchscreen0
    ------------[ cut here ]------------
    WARNING: CPU: 0 PID: 720 at drivers/clk/clk.c:727 clk_core_enable+0x2c/0xa4()
    Modules linked in: sc16is7xx snd_soc_uda1380
    CPU: 0 PID: 720 Comm: cat Not tainted 4.3.0-rc2+ #199
    Hardware name: LPC32XX SoC (Flattened Device Tree)
    Backtrace:
    [<>] (dump_backtrace) from [<>] (show_stack+0x18/0x1c)
    [<>] (show_stack) from [<>] (dump_stack+0x20/0x28)
    [<>] (dump_stack) from [<>] (warn_slowpath_common+0x90/0xb8)
    [<>] (warn_slowpath_common) from [<>] (warn_slowpath_null+0x24/0x2c)
    [<>] (warn_slowpath_null) from [<>] (clk_core_enable+0x2c/0xa4)
    [<>] (clk_core_enable) from [<>] (clk_enable+0x24/0x38)
    [<>] (clk_enable) from [<>] (lpc32xx_setup_tsc+0x18/0xa0)
    [<>] (lpc32xx_setup_tsc) from [<>] (lpc32xx_ts_open+0x14/0x1c)
    [<>] (lpc32xx_ts_open) from [<>] (input_open_device+0x74/0xb0)
    [<>] (input_open_device) from [<>] (evdev_open+0x110/0x16c)
    [<>] (evdev_open) from [<>] (chrdev_open+0x1b4/0x1dc)
    [<>] (chrdev_open) from [<>] (do_dentry_open+0x1dc/0x2f4)
    [<>] (do_dentry_open) from [<>] (vfs_open+0x6c/0x70)
    [<>] (vfs_open) from [<>] (path_openat+0xb4c/0xddc)
    [<>] (path_openat) from [<>] (do_filp_open+0x40/0x8c)
    [<>] (do_filp_open) from [<>] (do_sys_open+0x124/0x1c4)
    [<>] (do_sys_open) from [<>] (SyS_open+0x2c/0x30)
    [<>] (SyS_open) from [<>] (ret_fast_syscall+0x0/0x38)

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>