util_axis_fifo: Fix tkeep signal value when KEEP_EN is 0

[IstvanZsSzekely]

PR Description

Changed the tkeep default value to be 0 regardless of data width then KEEP_EN is set to 0.
Wasn't tested on any of the project level testbenches or HDL projects and they shouldn't be affected either.
Tested in the DMAC IP level testbenches found in this repository, fixes a compilation issue and the test as well.

PR Type

• Bug fix (change that fixes an issue)
• New feature (change that adds new functionality)
• Breaking change (has dependencies in other repos or will cause CI to fail)

PR Checklist

• I have followed the code style guidelines
• I have performed a self-review of changes
• I have compiled all hdl projects and libraries affected by this PR
• I have tested in hardware affected projects, at least on relevant boards
• I have commented my code, at least hard-to-understand parts
• I have signed off all commits from this PR
• I have updated the documentation (wiki pages, ReadMe files, Copyright etc)
• I have not introduced new Warnings/Critical Warnings on compilation
• I have added new hdl testbenches or updated existing ones