mtd: cfi_cmdset_0020: Deinline do_write_buffer, save 5316 bytes

This function compiles to 2554 bytes of machine code.
In C, the function is almost 200 lines long.

It has only one callsite, but forced inlining that much code
makes gcc generate significantly worse code. Let gcc itself decide
what to do.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
CC: David Woodhouse <David.Woodhouse@intel.com>
CC: Dan Carpenter <dan.carpenter@oracle.com>
CC: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
CC: linux-mtd@lists.infradead.org
CC: linux-kernel@vger.kernel.org
Signed-off-by: Brian Norris <computersforpeace@gmail.com>