NFC: microread: drop unused variable

In microread_i2c_irq_thread_fn 'client' set but not used

Cc: Lauro Ramos Venancio <lauro.venancio@openbossa.org>
Cc: Aloisio Almeida Jr <aloisio.almeida@openbossa.org>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>