ubi: Fix section mismatch

WARNING: vmlinux.o(.text+0x1f2a80): Section mismatch in reference from the variable __param_ops_mtd to the function .init.text:ubi_mtd_param_parse()
The function __param_ops_mtd() references
the function __init ubi_mtd_param_parse().
This is often because __param_ops_mtd lacks a __init
annotation or the annotation of ubi_mtd_param_parse is wrong.

Cc: Richard Weinberger <richard@nod.at>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>