staging: rtl8192u: Fix misspelling in variable name - Style

Fix the spelling mistake in variable name 'txhipower_threshhold'.
'threshhold' should be 'threshold'. Issue found by checkpatch.

This is a coding style change which should have no impact on runtime
code execution.

Signed-off-by: Kimberly Brown <kimbrownkd@gmail.com>
Reviewed-by: Vaishali Thakkar <vthakkar@vaishalithakkar.in>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>