net: renesas: rswitch: Add spin lock protection for irq {un}mask

Add spin lock protection for irq {un}mask registers' control.

After napi_complete_done() and this protection were applied,
a lot of redundant interrupts no longer occur.

For example: when "iperf3 -c <ipaddr> -R" on R-Car S4-8 Spider
 Before the patches are applied: about 800,000 times happened
 After the patches were applied: about 100,000 times happened

Fixes: 3590918b

 ("net: ethernet: renesas: Add support for "Ethernet Switch"")
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>