mtd: core: Drop duplicate NULL checks around nvmem_unregister()

mtd: core: Drop duplicate NULL checks around nvmem_unregister()

Since nvmem_unregister() checks for NULL, no need to repeat in
the caller. Drop duplicate NULL checks.

Acked-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20220220151527.17216-13-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>