hwrng: core - Simplify RNG switching from sysfs

If we attempt to use sysfs to change the current RNG in the usual
way i.e. issuing something like:

`echo 8a8a000.rng > /sys/devices/virtual/misc/hw_random/rng_current`

... it will fail because the code doesn't currently take the '\n'
into consideration.  Well, now it does.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>