UBI: block: Mark init-only symbol as __initdata

ubiblock_param_ops should be marked as __init as it's only used to set
a driver parameter on insertion time. This commit fixes the following:

  WARNING: drivers/mtd/built-in.o(.text+0x653ac): Section mismatch in
  reference from the variable ubiblock_param_ops to the function
  .init.text:ubiblock_set_param()

  The function ubiblock_param_ops() references the function __init
  ubiblock_set_param(). This is often because ubiblock_param_ops lacks a
  __init annotation or the annotation of ubiblock_set_param is wrong.

Given gcc errors if the struct is marked const __initdata, this commit
drops the const mark from it.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>