riscv: _isr_wrapper: get rid of the ASSUME_EQUAL() macro

This is really useful only for one case i.e. when testing against zero.
Do that test inline where it is needed and make the rest of the code
independent from the actual numerical value being tested to make code
maintenance easier if/when new cases are added.

Signed-off-by: Nicolas Pitre <npitre@baylibre.com>