nvmem: core: remove unneeded NULL check

"p" is the list iterator so it can't be NULL.  Static checkers complain
about this unnecessary check because we dereference the list iterator to
get the next item in the list so we'd be in trouble if it really was
NULL.  I have removed the check.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>