microblaze: Correct ffs regression for Microblaze

We are using generic implementation of ffs. This should
be part of Simon's commit 0413cfec

Here is warning message which this patch removes.

In file included from /tmp/u-boot-microblaze/include/common.h:38,
                 from cmd_mtdparts.c:87:
/tmp/u-boot-microblaze/include/linux/bitops.h:123:1: warning: "ffs" redefined
In file included from /tmp/u-boot-microblaze/include/linux/bitops.h:110,
                 from /tmp/u-boot-microblaze/include/common.h:38,
                 from cmd_mtdparts.c:87:
/tmp/u-boot-microblaze/include/asm/bitops.h:269:1:
warning: this is the location of the previous definition

Signed-off-by: Michal Simek <monstr@monstr.eu>