firmware: turris-mox-rwtm: fail probing when firmware does not support hwrng

firmware: turris-mox-rwtm: fail probing when firmware does not support hwrng

When Marvell's rWTM firmware, which does not support the GET_RANDOM
command, is used, kernel prints an error message
  hwrng: no data available
every 10 seconds.

Fail probing of this driver if the rWTM firmware does not support the
GET_RANDOM command.

Fixes: 389711b37493 ("firmware: Add Turris Mox rWTM firmware driver")
Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>