clk: stm32f4: avoid uninitialized variable access

The failure path in the newly added function tries to free an
uninitialized pointer:

drivers/clk/clk-stm32f4.c: In function 'stm32f4_rcc_init':
drivers/clk/clk-stm32f4.c:1106:4: error: 'gate' may be used uninitialized in this function [-Werror=maybe-uninitialized]

I'm adding an initialization to NULL here to make the kfree()
succeed, and I'm also rearranging the cleanup so that the
same kfree() is used for any error path, making the function
slightly more robust against newly introduced bugs in the
error handling.

Fixes: daf2d117 ("clk: stm32f4: Add lcd-tft clock")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Gabriel Fernandez <gabriel.fernandez@st.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>