sh: boards: Fix the cacography in irq.c

The word 'swtich' is wrong, so fix it.

Signed-off-by: Tang Bin <tangbin@cmss.chinamobile.com>
Reviewed-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Signed-off-by: Rich Felker <dalias@libc.org>