This patch enable parser_trx and disable mtdsplit_trx for mt76x8
subtarget.
The trx format is used only on Buffalo WCR-1166DS in mt76x8 subtarget
and the parser need to be switched to parser_trx to use the custom magic
number in the header for WCR-1166DS.

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>