clk: palmas: fix a possible NULL dereference

of_match_device could return NULL, and so cause a NULL pointer
dereference later.
Even if the probability of this case is very low, fixing it made
static analyzers happy.

Solving this with of_device_get_match_data made also code simplier.

Reported-by: coverity (CID 1324137)
Signed-off-by: LABBE Corentin <clabbe.montjoie@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>