arc: remove '__init' for get_hw_config_num_irq()

get_hw_config_num_irq() may be called by normal iss_model_init_smp()
which is a function pointer for 'init_smp' which may be called by
first_lines_of_secondary() which also need be normal too.

The related warning (with allmodconfig):

    MODPOST vmlinux.o
  WARNING: vmlinux.o(.text+0x5814): Section mismatch in reference from the function iss_model_init_smp() to the function .init.text:get_hw_config_num_irq()
  The function iss_model_init_smp() references
  the function __init get_hw_config_num_irq().
  This is often because iss_model_init_smp lacks a __init
  annotation or the annotation of get_hw_config_num_irq is wrong.

Signed-off-by: Chen Gang <gang.chen@asianux.com>