mtd: bcm47xxnflash: Add error handling for bcm47xxnflash_ops_bcm4706_ctl_cmd()

The function bcm47xxnflash_ops_bcm4706_cmd_ctrl() calls the function
bcm47xxnflash_ops_bcm4706_ctl_cmd(), but does not check its return value.
A proper implementation can be found in bcm47xxnflash_ops_bcm4706_write().

Add error log to the bcm47xxnflash_ops_bcm4706_ctl_cmd() via pr_err()
to prevent silent failure. The error Log funciton 'dev_err' is unsuitable
in this situation for it is hard to get device pointer.

Signed-off-by: Wentao Liang <vulab@iscas.ac.cn>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>