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

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

When building with clang 18 I see the following warning:
|       drivers/mtd/maps/physmap-versatile.c:209:25: warning: cast to smaller
|               integer type 'enum versatile_flashprot' from 'const void *' [-Wvoid-pointer-to-enum-cast]
|         209 |                 versatile_flashprot = (enum versatile_flashprot)devid->data;

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

Cast `devid->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/20230816-void-drivers-mtd-maps-physmap-versatile-v2-1-433a25272bfa@google.com