nvmem: remove unneeded IS_ENABLED(CONFIG_NVMEM) conditional

As you see in drivers/nvmem/Makefile, this C file is compiled only
when CONFIG_NVMEM is y or m.

So, IS_ENABLED(CONFIG_NVMEM) is always evaluated to 1 in this file.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>