staging:rtl8192u: Remove member variable rx_gain_range_max - Style

The structure 'dig' defines a member variable, (rx_gain_range_max)
which is initialised to the value 'DM_DIG_MAX', (a defined constant).
The variable is then used to test and set another variable. Since
the member rx_gain_range_max is never assigned any other value then
the constant 'DM_DIG_MAX' the code might as well simply use that
constant, rather the a member variable set to that constant.

The member variable has been removed and the constant used directly
in the code. This is a coding style change which should not impact
runtime code execution.

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