kernel: structs: define priq bitmap size via bits per long

Since a long is 64-bits on 64-bit architectures, and 32-bits
on 32-bit architectures, we can simplify the definition of
PRIQ_BITMAP_SIZE by defining it in terms of BITS_PER_LONG.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>