staging:rtl8192u: Rename EEPROM_TxPowerDiff - Style

Rename EEPROM_TxPowerDiff to EEPROM_TX_POWER_DIFF. This change clears
the checkpatch issue with CamelCase naming.

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

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