[LPC824] Fix serial interrupt issues

Basically, this is same fix as
https://github.com/mbedmicro/mbed/pull/633

* Fix issue by handling interrupt type (TxIrq and RxIrq)
* ISR now correctly refers INTSTAT instead of STAT to get interrupt
cause
* Disable interrupt when updating vector table
* Tested by issue #616 test cases and serial_interrupt test