igc: Increase rx_buffer_size local variable to 32b

Apply 6987c47569b377f4b6eba9966afdedfb1b39fca8 to igc. This is not
expected to have any benefit on current parts with current observed PBA
sizes but will avoid surprises if they are increased in future chips.

Approved by: grehan
Differential Revision: https://reviews.freebsd.org/D35217

(cherry picked from commit 9b88ecd674ac292cb6d7ab644595091edd6ec87b)