serial8250-em: clk_get() IS_ERR() error handling fix

Update the 8250_em driver to correctly handle the case
where no clock is associated with the device.

The return value of clk_get() needs to be checked with
IS_ERR() to avoid NULL pointer referencing.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>