rtwn(4): change type for Tx power values (RTL8192C / RTL8188EU).

Tx power values can easily fit into uint8_t + only 8 bits are written
to registers; values may overflow only in case if ROM contains
malformed data (but limit is checked anyway).

Tested with RTL8188CUS, dev.rtwn.1.debug=0x2000 (no changes).