riscv: fix styling in ucontext header

Change the two comments in ucontext.h by getting them up to
the coding style proposed by torvalds.

Signed-off-by: Cleo John <waterdev@galaxycrow.de>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20221010182848.GA28029@watet-ms7b87
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>