i2c: brcmstb: Fix brcmstd_send_i2c_cmd condition

The brcmstb_send_i2c_cmd currently has a condition that is (CMD_RD ||
CMD_WR) which always evaluates to true, while the obvious fix is to test
whether the cmd variable passed as parameter holds one of these two
values.

Fixes: dd1aa252 ("i2c: brcmstb: Add Broadcom settop SoC i2c controller driver")
Reported-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>