mfd: sun6i-prcm: Fix build warning for non-OF configurations

When CONFIG_OF is disabled, we get a harmless warning about an
unused variable:

drivers/mfd/sun6i-prcm.c: In function 'sun6i_prcm_probe':
drivers/mfd/sun6i-prcm.c:151:22: error: unused variable 'np' [-Werror=unused-variable]

Remove the variable and open-code the value in the only place
it is used, so it can get left out as well without CONFIG_OF.

Fixes: a05a2e79 ("mfd: sun6i-prcm: Allow to compile with COMPILE_TEST")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>