sh_eth: workaround for spurious ECI interrupt

At least on Renesas R8A7778, EESR.ECI interrupt seems to fire regardless of its
mask in EESIPR register. I can 100% reproduce it with the following scenario:
target is booted with 'ip=on' option, and so IP-Config opens SoC Ether device
but doesn't get a proper reply and then succeeds with on-board SMC chip; then
I login and try to bring up the SoC Ether device with 'ifconfig', and I get
an ECI interrupt once request_irq() is called by sh_eth_open() (while interrupt
mask in EESIPR register is all 0), if that interrupt is accompanied by a pending
EESR.FRC (frame receive completion) interrupt, I get kernel oops in sh_eth_rx()
because sh_eth_ring_init() hasn't been called yet!

The solution I worked out is the following: in sh_eth_interrupt(), mask the
interrupt status from EESR register with the interrupt mask from EESIPR register
in order not to handle the disabled interrupts -- but forcing EESIPR.M_ECI bit
in this mask set because we always need to fully handle EESR.ECI interrupt in
sh_eth_error() in order to quench it (as it doesn't get cleared by just writing
1 to the this bit as all the other interrupts).

While at it, remove unneeded initializer for 'intr_status' variable and give it
*unsigned long* type, matching the type of sh_eth_read()'s result; fix comment.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Max Filippov <max.filippov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>