MIPS: msi-octeon: eliminate kernel-doc warnings

Rearrange kernel-doc notation for 2 functions to eliminate
kernel-doc warnings. Use Return: notation for the function
return value description. Add function short descriptions
for both functions.
Correct 2 typos.

Fixes these kernel-doc warnings:

msi-octeon.c:49: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * Called when a driver request MSI interrupts instead of the
msi-octeon.c:49: warning: missing initial short description on line:
 * Called when a driver request MSI interrupts instead of the
msi-octeon.c:62: warning: No description found for return value of 'arch_setup_msi_irq'
msi-octeon.c:189: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 * Called when a device no longer needs its MSI interrupts. All
msi-octeon.c:189: warning: missing initial short description on line:
 * Called when a device no longer needs its MSI interrupts. All

Fixes: e8635b48 ("MIPS: Add Cavium OCTEON PCI support.")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Aditya Srivastava <yashsri421@gmail.com>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: linux-mips@vger.kernel.org
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>