staging:rtl8192u: Refactor enum dm_dig_pd_th_e - Style

The enumerated type dm_dig_pd_th_e is never actually used as
the type for the two variables which use the constants, which the
enumeration defines. This omission removes the possibility of taking
advantage of compiler type checking.

To correct this the two member variables, (curpd_thstate & prepd_thstate)
have been changed to use the type enum dig_pkt_detection_threshold rather
then u8.

Additionally the enum's declaration has been moved above the dig
structure, where the type is used, the 'typedef' has been removed to
clear the checkpatch issue with defining new types, and the value
'DIG_PD_MAX' has been removed from the enumeration, since it is never
used in code.

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