selftests/intel_pstate: fix operand expected error

selftests/intel_pstate: fix operand expected error

Running "make kselftest TARGETS=intel_pstate" results in
the following errors:
- ./run.sh: line 90: / 1000: syntax error: operand expected
(error token is "/ 1000")
- ./run.sh: line 92: / 1000: syntax error: operand expected
(error token is "/ 1000")

This fix allows to have cross-platform compatibility when
using arithmetic expression with command substitutions.

Link: https://lore.kernel.org/r/f37df23888cd5ea6b3976f19d3e25796129dd090.1730141362.git.alessandro.zanni87@gmail.com
Signed-off-by: Alessandro Zanni <alessandro.zanni87@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>