fix: read ICSR volatile in SCB::vect_active()

[jakobjung10]

SCB::vect_active() now reads ICSR with a volatile access, preventing the load from being narrowed to a byte read that faults on word-only ICSR models.

[jonathanpallant]

That's a plain read not a volatile read?

[jakobjung10]

read() in (*SCB::PTR).icsr.read() internally calls self.register.get() from volatile-register, which again internally calls ptr::read_volatile(self.value.get()) from vcell.
Therefore, it is a volatile read.

Also, calling read() aligns more with the remaining scb module, as the direct call of ptr::read(&(*SCB::PTR).icsr as *const _ as *const u32) is actually the outlier.

[jonathanpallant]

oof, fair point - my mistake!