selftests/powerpc: Make the test check in eeh-basic.sh posix compliant

The == operand is a bash extension, thus this will fail on Ubuntu
with:
  ./eeh-basic.sh: 89: test: 2: unexpected operator

As the /bin/sh on Ubuntu is pointed to DASH.

Use -eq to fix this posix compatibility issue.

Fixes: 996f9e0f ("selftests/powerpc: Fix eeh-basic.sh exit codes")
Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Reviewed-by: Frederic Barrat <fbarrat@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201228043459.14281-1-po-hsu.lin@canonical.com