mtd: rawnand: fix -Wvoid-pointer-to-enum-cast warning

When building with clang 18 I see the following warning:
|       drivers/mtd/nand/raw/vf610_nfc.c:853:17: warning: cast to smaller integer
|               type 'enum vf610_nfc_variant' from 'const void *' [-Wvoid-pointer-to-enum-cast]
|         853 |         nfc->variant = (enum vf610_nfc_variant)of_id->data;

This is due to the fact that `of_id->data` is a void* while `enum vf610_nfc_variant`
has the size of an int.

Cast `of_id->data` to a uintptr_t to silence the above warning for clang
builds using W=1.

Link: https://github.com/ClangBuiltLinux/linux/issues/1910
Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Justin Stitt <justinstitt@google.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20230817-void-drivers-mtd-nand-raw-vf610_nfc-v2-1-870a7c948c44@google.com