spi: mpc8xxx: Simplify logic a bit

spi: mpc8xxx: Simplify logic a bit

We do nothing in the loop if the "not empty" event was not detected. To
simplify the logic, check if this is the case, and skip the execution of
the loop early to reduce the nesting level and flag checking.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
Acked-by: Jagan Teki <jagan@amarulasolutions.com>