wifi: rt2x00: remove redundant check if u8 array element is less than zero

wifi: rt2x00: remove redundant check if u8 array element is less than zero

The check on vga_gain[ch_idx] being less than zero is always false since
vga_gain is a u8 array. Clean up the code by removing the check and the
following assignment. Cleans up clang scan build warning:

drivers/net/wireless/ralink/rt2x00/rt2800lib.c:9704:26: warning:
result of comparison of unsigned expression < 0 is always
false [-Wtautological-unsigned-zero-compare]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230920121240.120455-1-colin.i.king@gmail.com