ieee802154/adf7242: check status of adf7242_read_reg

ieee802154/adf7242: check status of adf7242_read_reg

Clang static analysis reports this error

adf7242.c:887:6: warning: Assigned value is garbage or undefined
        len = len_u8;
            ^ ~~~~~~

len_u8 is set in
       adf7242_read_reg(lp, 0, &len_u8);

When this call fails, len_u8 is not set.

So check the return code.

Fixes: 7302b9d90117 ("ieee802154/adf7242: Driver for ADF7242 MAC IEEE802154")
Signed-off-by: Tom Rix <trix@redhat.com>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Link: https://lore.kernel.org/r/20200802142339.21091-1-trix@redhat.com
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>