mfd: madera: Work around false-positive -Wininitialized warning

mfd: madera: Work around false-positive -Wininitialized warning

clang-21 warns about one uninitialized variable getting dereferenced
in madera_dev_init:

drivers/mfd/madera-core.c:739:10: error: variable 'mfd_devs' is uninitialized when used here [-Werror,-Wuninitialized]
  739 |                               mfd_devs, n_devs,
      |                               ^~~~~~~~
drivers/mfd/madera-core.c:459:33: note: initialize the variable 'mfd_devs' to silence this warning
  459 |         const struct mfd_cell *mfd_devs;
      |                                        ^
      |                                         = NULL

The code is actually correct here because n_devs is only nonzero
when mfd_devs is a valid pointer, but this is impossible for the
compiler to see reliably.

Change the logic to check for the pointer as well, to make this easier
for the compiler to follow.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20250807071932.4085458-1-arnd@kernel.org
Signed-off-by: Lee Jones <lee@kernel.org>