hwrng: imx-rngc - reasonable timeout for selftest

Set a more reasonable timeout for the rngc selftest.

According to the reference manual, "The self test takes approximately
29,000 cycles to complete." The lowest possible frequency of the rngc
peripheral clock is 33.25MHz, the selftest would then take about 872us.

2.5ms should be enough for the selftest timeout.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>