mmc: block: init force_ro sysfs attribute

To avoid lockdep warnings:

BUG: key dc90a520 not in .data!
------------[ cut here ]------------
WARNING: at /home/rabin/kernel/arm/kernel/lockdep.c:2701 sysfs_add_file_mode+0x4c/0xb0()
Modules linked in:
[<c004b5d8>] (unwind_backtrace+0x0/0xe4) from [<c0074f20>] (warn_slowpath_common+0x4c/0x64)
[<c0074f20>] (warn_slowpath_common+0x4c/0x64) from [<c0074f50>] (warn_slowpath_null+0x18/0x1c)
[<c0074f50>] (warn_slowpath_null+0x18/0x1c) from [<c0157fec>] (sysfs_add_file_mode+0x4c/0xb0)
[<c0157fec>] (sysfs_add_file_mode+0x4c/0xb0) from [<c02d61e4>] (mmc_add_disk+0x40/0x64)
[<c02d61e4>] (mmc_add_disk+0x40/0x64) from [<c02d64cc>] (mmc_blk_probe+0x188/0x1fc)
[<c02d64cc>] (mmc_blk_probe+0x188/0x1fc) from [<c02ce820>] (mmc_bus_probe+0x14/0x18)
...

Signed-off-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Chris Ball <cjb@laptop.org>