ds1wm: should check for IS_ERR(clk) instead of NULL

On the error condition clk_get() returns ERR_PTR(..), so checking for NULL
doesn't work.  ds1wm module causes a kernel oops when ds1wm clock isn't
registered.

This patch converts NULL check to IS_ERR(), plus uses PTR_ERR()
for the return code.

Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>
Acked-by: Evgeniy Polyakov <johnpol@2ka.mipt.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>