nfc: nxp-nci: store __be16 value in __be16 variable

Use a __be16 variable to store the big endian value of header in
nxp_nci_i2c_fw_read().

Flagged by Sparse as:

 .../i2c.c:113:22: warning: cast to restricted __be16

No functional changes intended.
Compile tested only.

Signed-off-by: Simon Horman <horms@kernel.org>
Reviewed-by: Sridhar Samudrala <sridhar.samudrala@intel.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>