lib: simplify sbi_fifo_inplace_update()

Don't assign an unused value to variable index.

Use operator '-=' where applicable.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Xiang W <wxjstz@126.com>