clk: zynqmp: fix compile testing without ZYNQMP_FIRMWARE

clk: zynqmp: fix compile testing without ZYNQMP_FIRMWARE

When the firmware code is disabled, the incomplete error handling
in the clk driver causes compile-time warnings:

drivers/clk/zynqmp/pll.c: In function 'zynqmp_pll_recalc_rate':
drivers/clk/zynqmp/pll.c:147:29: error: 'fbdiv' is used uninitialized [-Werror=uninitialized]
  147 |         rate =  parent_rate * fbdiv;
      |                 ~~~~~~~~~~~~^~~~~~~
In function 'zynqmp_pll_get_mode',
    inlined from 'zynqmp_pll_recalc_rate' at drivers/clk/zynqmp/pll.c:148:6:
drivers/clk/zynqmp/pll.c:61:27: error: 'ret_payload' is used uninitialized [-Werror=uninitialized]
   61 |         return ret_payload[1];
      |                ~~~~~~~~~~~^~~
drivers/clk/zynqmp/pll.c: In function 'zynqmp_pll_recalc_rate':
drivers/clk/zynqmp/pll.c:53:13: note: 'ret_payload' declared here
   53 |         u32 ret_payload[PAYLOAD_ARG_CNT];
      |             ^~~~~~~~~~~
drivers/clk/zynqmp/clk-mux-zynqmp.c: In function 'zynqmp_clk_mux_get_parent':
drivers/clk/zynqmp/clk-mux-zynqmp.c:57:16: error: 'val' is used uninitialized [-Werror=uninitialized]
   57 |         return val;
      |                ^~~

As it was apparently intentional to support this for compile testing
purposes, change the code to have just enough error handling for the
compiler to not notice the remaining bugs.

Fixes: 21f237534661 ("clk: zynqmp: Drop dependency on ARCH_ZYNQMP")
Co-developed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/f1c4e8c903fe2d5df5413421920a56890a46387a.1624356908.git.michal.simek@xilinx.com
Signed-off-by: Stephen Boyd <sboyd@kernel.org>