arch: Fix bounds checking for dynamic shared interrupts

Fixes z_isr_install() and z_isr_uninstall() routines to check the isr
table index against the correct number of isr table entries. This
prevents out-of-bounds isr table accesses when
CONFIG_GEN_IRQ_START_VECTOR is greater than zero, such as on the ARC
architecture.

Coverity-CID: 347187
Coverity-CID: 347189
Coverity-CID: 347097
Coverity-CID: 347203
Coverity-CID: 347210

Signed-off-by: Maureen Helm <maureen.helm@analog.com>