tpm: cleanup tpm_tis_remove()

Created a local variable pointing to the INT_ENABLE_x register. The
expression clearing INT_ENABLE_x.globalIntEnable is unreadable and
hard to modify without surpassing the 80 char boundary.

Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Tested-by: Christophe Ricard <christophe-h.ricard@st.com>