gianfar: don't duplicate gfar_error()

It was hardly necessary to repeat most of the code from gfar_error() in
gfar_interrupt(), especially having some inconsistencies between the two.
So, make the gfar_interrupt() just call gfar_error(), and not acknowledge
the interrupts itself as gfar_{receive/transmit/error}() do it anyway.
While at it, also clarify/cleanup debug messages in gfar_error()...

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>