cappuccino/lsu: stay in WRITE state on store_buffer_write

Apart from being one clock cycle faster, this also avoids connecting
the (potentially critical) dbus_ack_i signal to the read enable
input of the store buffer.

In addition to that it also fixes a bug where the lsu_valid
signal would be detached from the storebuffer logic when
the state machine would transition from WRITE->IDLE->WRITE.