asm-generic: remove a broken and needless ifdef conditional

Commit 527701e ("lib: Add a generic version of devmem_is_allowed()")
introduces the config symbol GENERIC_LIB_DEVMEM_IS_ALLOWED, but then
falsely refers to CONFIG_GENERIC_DEVMEM_IS_ALLOWED (note the missing LIB
in the reference) in ./include/asm-generic/io.h.

Luckily, ./scripts/checkkconfigsymbols.py warns on non-existing configs:

GENERIC_DEVMEM_IS_ALLOWED
Referencing files: include/asm-generic/io.h

The actual fix, though, is simply to not to make this function declaration
dependent on any kernel config. For architectures that intend to use
the generic version, the arch's 'select GENERIC_LIB_DEVMEM_IS_ALLOWED' will
lead to picking the function definition, and for other architectures, this
function is simply defined elsewhere.

The wrong '#ifndef' on a non-existing config symbol also always had the
same effect (although more by mistake than by intent). So, there is no
functional change.

Remove this broken and needless ifdef conditional.

Fixes: 527701e ("lib: Add a generic version of devmem_is_allowed()")
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

include/asm-generic/io.h

@@ -1125,9 +1125,7 @@ static inline void memcpy_toio(volatile void __iomem *addr, const void *buffer,
 }
 #endif
 
-#ifndef CONFIG_GENERIC_DEVMEM_IS_ALLOWED
 extern int devmem_is_allowed(unsigned long pfn);
-#endif
 
 #endif /* __KERNEL__ */