Input: ads7846 - drop unneeded asm/irq.h include

This is essentially a revert of:

	3ac8bf07 ("[PATCH] ads7846: sparc32 warning fix")

By now enable_irq() and disable_irq() are properly defined in
linux/interrupt.h and we do not need to pull in architecture-specific
bits.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>