cpu: Fix the type of the effective mem request size

cpu: Fix the type of the effective mem request size

A memory request size can be larger than 255 bytes (e.g.
SVE with 2048-bit vector length) which could cause overflow
in the 'uint8_t effSize' variable.

Change-Id: I77e0d02a49ea7f81cacfa5be7e4ae40434af3109
Reviewed-by: Giacomo Gabrielli <giacomo.gabrielli@arm.com>
Signed-off-by: Giacomo Gabrielli <giacomo.gabrielli@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/19175
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>