[ARM] 5451/1: ep93xx:core.c: build warning fix

Fix trivial build warning due to incompatible pointer type.

ep93xx_timer_interrupt() has the wrong return type causing a
warning during the build.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>