Two fixes from Mike Smith: (1) use -m install of --mode in install commands,...

Two fixes from Mike Smith: (1) use -m install of --mode in install commands, (2) fix typo in stm32_exti.h introduced with the STM32 F3 port

git-svn-id: svn://svn.code.sf.net/p/nuttx/code/trunk@5701 42af7a65-404d-4744-a932-0658087f49c3