blackfin: asm: fix blackfin.h broken

do{...}while makes no sense in __ASSEMBLY__ code paths.  now
kernels fail to build:
arch/blackfin/mach-bf561/atomic.S: Assembler messages:
arch/blackfin/mach-bf561/atomic.S:48: Error: syntax error. Input text
was do.
arch/blackfin/mach-bf561/atomic.S:48: Error:
arch/blackfin/mach-bf561/atomic.S:48: Error: syntax error. Input text
was }.
arch/blackfin/mach-bf561/atomic.S:48: Error:
arch/blackfin/mach-bf561/atomic.S:53: Error: syntax error. Input text
was do.
arch/blackfin/mach-bf561/atomic.S:53: Error:
arch/blackfin/mach-bf561/atomic.S:53: Error: syntax error. Input text
was }.

Signed-off-by: Bob Liu <lliubbo@gmail.com>