sama3 sam_serial.c USART selection fixes

[adamfeuer]

Summary

• Fixes bugs in USART console selection
• Fixes a few typos in the SAMA5D27 README file

Impact

• SAMA5 USART console selection should work correctly now

Limitations / TODO

• Note! activating UART2 as a console on the SAMA5D27 causes compilation errors because interrupts aren't working right on that chip.
• SAMA5D27 console input still not working due to incomplete port – hopefully more changes will come in a future PR

Detail

• Looks like these were typos when the port was created

Testing

• Manual – compile for a SAMA5D27-XULT board. With UART2 enabled, NuttX doesn't compile.