sh: correct definitions to access stack pointers

A definition like:

#define regs_return_value(regs)	((regs)->regs[0])

called with regs_return_value(foo) will be preprocessed to:

((foo)->foo[0])
        ^^^
So to fix this to ensure the preprocessor compiles such calls correctly.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>