clk: tegra20: fix gcc-7 constant overflow warning

clk: tegra20: fix gcc-7 constant overflow warning

Older gcc versions get confused by comparing a u32 value to a negative
constant in a switch()/case block:

drivers/clk/tegra/clk-tegra20.c: In function 'tegra20_clk_measure_input_freq':
drivers/clk/tegra/clk-tegra20.c:581:2: error: case label does not reduce to an integer constant
  case OSC_CTRL_OSC_FREQ_12MHZ:
  ^~~~
drivers/clk/tegra/clk-tegra20.c:593:2: error: case label does not reduce to an integer constant
  case OSC_CTRL_OSC_FREQ_26MHZ:

Make the constants unsigned instead.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20230227085914.2560984-1-arnd@kernel.org
Signed-off-by: Stephen Boyd <sboyd@kernel.org>