staging: vt6655: Use named constants when checking preamble type

Compare `preamble_type` to a predefined constant, `PREAMBLE_SHORT`,
instead of a literal in `bb_get_frame_time` and `vnt_get_phy_field`
functions. Thanks to this change, it is clear that we check
the type of a preamble, not just compare it against an arbitrary value.
Remove a redundant comment.

Signed-off-by: Karolina Drobnik <karolinadrobnik@gmail.com>
Link: https://lore.kernel.org/r/20211021092753.511863-1-karolinadrobnik@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>