Fix unused variable warning in rockchip's rk_spi.c

With clang 15, the following -Werror warning is produced:

sys/arm64/rockchip/rk_spi.c:229:6: error: variable 'cnt' set but not used [-Werror,-Wunused-but-set-variable]
        int cnt = 0;
            ^

The 'cnt' variable was in rk_spi.c when it was first added, but it
appears to have been a debugging aid that has never been used, so remove
it.

MFC after: 3 days

(cherry picked from commit 0fe8ab6851687dc7249e3c5b5df3d6723d1c3e7a)