pflash_cfi01: Fix warning caused by unreachable code

Report from smatch:
hw/pflash_cfi01.c:431 pflash_write(180) info: ignoring unreachable code.

Instead of removing the return statement after the switch statement,
the patch replaces the return statements in the switch statement by
break statements. Other switch statements in the same code do it also
like that.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>