serial: sprd: check for NULL after calling devm_clk_get

In platforms which does not use CLK framework (HAVE_CLK not set), the
clk_* functions return NULL instead of an error. This patch handles that
scenario.

Signed-off-by: Fernando Guzman Lugo <fernando.guzman.lugo@intel.com>
Signed-off-by: Chunyan Zhang <chunyan.zhang@spreadtrum.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>