rtlwifi: rtl8192cu: Fix value set in descriptor

In the process of converting the bit manipulation macros were converted
to use GENMASK(), the compiler reported a value too big for the field.
The offending statement was trying to write 0x100 into a 5-bit field.
An accompaning comment says to set bit 3, thus the code is changed
appropriately.

This error has been in the driver since its initial submission.

Fixes: 29d00a3e ("rtlwifi: rtl8192cu: Add routine trx")
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>