eradicate bashisms in scripts/patch-kernel

Silence a remaining annoying (or worse, irritating - "is my entire patched tree
broken now!?") bashism-related message that occurs when /bin/sh is configured
to instead deploy dash, a POSIX-compliant shell, as is the pretty much
standard case on e.g. Debian.

Current kernel version is 2.6.38 ( Flesh-Eating Bats with Fangs)
===>  linux-2.6.38.patch-kernel_test/scripts/patch-kernel: line 253: [: =: unary operator expected  <===
cannot find patch file: patch-2.6.39

Signed-off-by: Andreas Mohr <andi@lisas.de>
Signed-off-by: Michal Marek <mmarek@suse.cz>