clk: axi-clkgen: Remove sometimes impossible check

The size of unsigned long on 64-bit architectures is equal to the
size of u64, so this check is impossible there. This throws off
static checkers:

drivers/clk/clk-axi-clkgen.c:331 axi_clkgen_recalc_rate() warn:
impossible condition '(tmp > (~0)) => (0-u64max > u64max)'

Let's change this code to use min_t() instead so that we
get the same effect on architectures where sizeof(unsigned long)
doesn't equal sizeof(u64).

Cc: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>