staging:rtl8192u: Rename EEPROM_PwDiff - Style

Rename the constant EEPROM_PwDiff to EEPROM_PW_DIFF. This change clears
the checkpatch issue with CamelCase naming.

This is a coding style change which should not impact on runtime
code execution.

Signed-off-by: John Whitmore <johnfwhitmore@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>