rtwn: correct TRXDMA_CTRL_QMAP mask for USB chips

[vexeduxr]

Summary

For USB chips, only the first three bits are preserved in the
TRXDMA_CTRL register.

Swap out rtwn_get_qmap for rtwn_set_qmap, which just writes the "QMAP"
value itself. This allows both the USB and PCI implementations to use
their own mask.

Tested on the RTL8723BU and the RTL8188EU.

[vexeduxr]

I think defining R92C_TRXDMA_CTRL_QMAP_M differently depending on the header can cause a bit of confusion, but I didn't want to break the naming conventions...

Edit: Note that not clearing the extra bit doesn't seem to make a difference for the USB chips. So I'm okay with just leaving this as is too...

For convenience, the vendor drivers I checked:

rtl8192cu: https://github.com/pvaret/rtl8192cu-fixes/blob/e5c01a328d2a1/hal/rtl8192c/usb/usb_halinit.c#L855
rtl8192eu: https://github.com/Mange/rtl8192eu-linux-driver/blob/da21d44a40690/hal/rtl8192e/rtl8192e_hal_init.c#L3240
rtl8188eu: https://github.com/lwfinger/rtl8188eu/blob/f5d1c8df2e2d8/hal/usb_halinit.c#L224
rtl8812au: https://github.com/aircrack-ng/rtl8812au/blob/734485506a30d/hal/rtl8812a/usb/usb_halinit.c#L618
rtl8821au: https://github.com/morrownr/8821au-20210708/blob/623d08cb49ba4/hal/rtl8812a/usb/usb_halinit.c#L606