i2c: omap: Fix error handling for clk_get()

Otherwise we can get the following if the fck alias is missing:

Unable to handle kernel paging request at virtual address fffffffe
...
PC is at clk_get_rate+0x8/0x10
LR is at omap_i2c_probe+0x278/0x6ec
...
[<c056eb08>] (clk_get_rate) from [<c06f4f08>] (omap_i2c_probe+0x278/0x6ec)
[<c06f4f08>] (omap_i2c_probe) from [<c0610944>] (platform_drv_probe+0x50/0xb0)
[<c0610944>] (platform_drv_probe) from [<c060e900>] (driver_probe_device+0x264/0x2ec)
[<c060e900>] (driver_probe_device) from [<c060cda0>] (bus_for_each_drv+0x70/0xb8)
[<c060cda0>] (bus_for_each_drv) from [<c060e5b0>] (__device_attach+0xcc/0x13c)
[<c060e5b0>] (__device_attach) from [<c060db10>] (bus_probe_device+0x88/0x90)
[<c060db10>] (bus_probe_device) from [<c060df68>] (deferred_probe_work_func+0x4c/0x14c)

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>