staging: rtl8188eu: use prefix decrement operator on trycnt variable

Modify trycnt condition in the do/while loop of rtw_hal_set_hwreg to use
a prefix decrement operator instead of postfix. As an unsigned 8-bit
integer, this value would actually be 255 after the end of the do/while
loop, which is almost certainly not what was intended. It is more
reasonable to assume a loop counter should be zero at the end of all
loop iterations. Indeed, the line following the loop previously contained
an if statement which assumed trycnt was 0, and therefore was never
triggered.

Signed-off-by: Phillip Potter <phil@philpotter.co.uk>
Link: https://lore.kernel.org/r/20210615223607.13863-3-phil@philpotter.co.uk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8188eu/hal/usb_halinit.c

@@ -1657,7 +1657,7 @@ void rtw_hal_set_hwreg(struct adapter *Adapter, u8 variable, u8 *val)
 				do {
 					if (!(usb_read32(Adapter, REG_RXPKT_NUM) & RXDMA_IDLE))
 						break;
-				} while (trycnt--);
+				} while (--trycnt);
 
 				/* RQPN Load 0 */
 				usb_write16(Adapter, REG_RQPN_NPQ, 0x0);