cpu: Replace empty byteEnable check with Request::isMasked

cpu: Replace empty byteEnable check with Request::isMasked

This should be the interface to be used to check if the request
has some masked bytes.

JIRA: https://gem5.atlassian.net/browse/GEM5-196

Change-Id: I1ab5fd266c7b63a928aada32ae6d4f7fa915f2b6
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23523
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>