PNP: isapnp: use str_plural() to simplify the code

Use the string choice helper function str_plural() to simplify the code.

Signed-off-by: Xichao Zhao <zhao.xichao@vivo.com>
Link: https://patch.msgid.link/20250818082451.497935-1-zhao.xichao@vivo.com


Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>