habanalabs: add missing error check in sysfs clk_freq_mhz_show

Add a missing error check in the sysfs show functions for
clk_max_freq_mhz and clk_cur_freq_mhz_show.

Signed-off-by: Tomer Tayar <ttayar@habana.ai>
Reviewed-by: Oded Gabbay <ogabbay@kernel.org>
Signed-off-by: Oded Gabbay <ogabbay@kernel.org>